Cherry-pick of API for generating burn txs in mobilecoind (#1872)

This conflict was resolved:

```
diff --cc mobilecoind/src/payments.rs
index 6424c3fd,c3b6f0ed..00000000
--- a/mobilecoind/src/payments.rs
+++ b/mobilecoind/src/payments.rs
@@@ -867,18 -876,17 +876,32 @@@ impl<T: BlockchainConnection + UserTxCo
              fog_resolver_factory(&fog_uris).map_err(Error::Fog)?
          };

++<<<<<<< HEAD
 +        // TODO: Use RTH memo builder, optionally?
 +
 +        let fee_amount = Amount::new(fee, token_id);
 +
 +        // Create tx_builder.
 +        let mut tx_builder = TransactionBuilder::new(
 +            block_version,
 +            fee_amount,
 +            fog_resolver,
 +            EmptyMemoBuilder::default(),
 +        )
 +        .map_err(|err| Error::TxBuild(format!("Error cretaing TransactionBuilder: {}", err)))?;
++=======
+         // Create tx_builder.
+         // TODO (GH #1522): Use RTH memo builder, optionally?
+         let memo_builder: Box<dyn MemoBuilder + Send + Sync> =
+             opt_memo_builder.unwrap_or_else(|| Box::new(EmptyMemoBuilder::default()));
+
+         let fee_amount = Amount::new(fee, token_id);
+         let mut tx_builder =
+             TransactionBuilder::new_with_box(block_version, fee_amount, fog_resolver, memo_builder)
+                 .map_err(|err| {
+                     Error::TxBuild(format!("Error creating transaction builder: {}", err))
+                 })?;
++>>>>>>> d991eee... API for generating burn txs in mobilecoind (#1872)

          // Unzip each vec of tuples into a tuple of vecs.
          let mut rings_and_proofs: Vec<(Vec<TxOut>, Vec<TxOutMembershipProof>)> = rings
```

mobilecoind/api/proto/mobilecoind_api.proto

@@ -44,6 +44,7 @@ service MobilecoindAPI {
     rpc GenerateOptimizationTx (GenerateOptimizationTxRequest) returns (GenerateOptimizationTxResponse) {}
     rpc GenerateTransferCodeTx (GenerateTransferCodeTxRequest) returns (GenerateTransferCodeTxResponse) {}
     rpc GenerateTxFromTxOutList (GenerateTxFromTxOutListRequest) returns (GenerateTxFromTxOutListResponse) {}
+    rpc GenerateBurnRedemptionTx (GenerateBurnRedemptionTxRequest) returns (GenerateBurnRedemptionTxResponse) {}
     rpc SubmitTx (SubmitTxRequest) returns (SubmitTxResponse) {}
 
     // Databases
@@ -563,6 +564,49 @@ message GenerateTxFromTxOutListResponse {
     TxProposal tx_proposal = 1;
 }
 
+// Generate a burn redemption transaction proposal object.
+// Notes:
+// - Sum of inputs needs to be greater than or equal to the burn amount and fee.
+// - The set of inputs to use would be chosen automatically by mobilecoind.
+// - The fee field could be set to zero, in which case mobilecoind would try and choose a fee.
+message GenerateBurnRedemptionTxRequest {
+    // Monitor id sending the funds.
+    bytes sender_monitor_id = 1;
+
+    // Subaddress to return change to.
+    uint64 change_subaddress = 2;
+
+    // List of UnspentTxOuts to be spent by the transaction.
+    // All UnspentTxOuts must belong to the same sender_monitor_id.
+    // mobilecoind would choose a subset of these inputs to construct the transaction.
+    // Total input amount must be >= burn amount + fee.
+    repeated UnspentTxOut input_list = 3;
+
+    // Amount to be burnt. This excludes change and fee.
+    uint64 burn_amount = 4;
+
+    // Fee (setting to 0 causes mobilecoind to choose a value).
+    // The value used can be checked (but not changed) in tx_proposal.tx.prefix.fee
+    uint64 fee = 5;
+
+    // Tombstone block (setting to 0 causes mobilecoind to choose a value).
+    // The value used can be checked (but not changed) in tx_proposal.tx.prefix.tombstone_block
+    uint64 tombstone = 6;
+
+    // Token id to use for the transaction.
+    uint64 token_id = 7;
+
+    // Optional 64 bytes of data to include in the burn redemption memo that is attached to the burn TxOut.
+    // If not provided zeros will be used.
+    bytes redemption_memo = 8;
+
+    // Enable RTH destination memo.
+    bool enable_destination_memo = 9;
+}
+message GenerateBurnRedemptionTxResponse {
+    TxProposal tx_proposal = 1;
+}
+
 // Submits a transaction to the network.
 message SubmitTxRequest {
     TxProposal tx_proposal = 1;

mobilecoind/src/payments.rs

@@ -24,7 +24,7 @@ use mc_transaction_core::{
     Amount, BlockIndex, BlockVersion, TokenId,
 };
 use mc_transaction_std::{
-    ChangeDestination, EmptyMemoBuilder, InputCredentials, TransactionBuilder,
+    ChangeDestination, EmptyMemoBuilder, InputCredentials, MemoBuilder, TransactionBuilder,
 };
 use mc_util_uri::FogUri;
 use rand::Rng;
@@ -221,6 +221,8 @@ impl<T: BlockchainConnection + UserTxConnection + 'static, FPR: FogPubkeyResolve
     /// * `outlays` - Output amounts and recipients.
     /// * `opt_fee` - Transaction fee in picoMOB. If zero, defaults to MIN_FEE.
     /// * `opt_tombstone` - Tombstone block. If zero, sets to default.
+    /// * `opt_memo_builder` - Optional memo builder to use instead of the
+    ///   default one (EmptyMemoBuilder).
     pub fn build_transaction(
         &self,
         sender_monitor_id: &MonitorId,
@@ -230,6 +232,7 @@ impl<T: BlockchainConnection + UserTxConnection + 'static, FPR: FogPubkeyResolve
         outlays: &[Outlay],
         opt_fee: u64,
         opt_tombstone: u64,
+        opt_memo_builder: Option<Box<dyn MemoBuilder + 'static + Send + Sync>>,
     ) -> Result<TxProposal, Error> {
         let logger = self.logger.new(o!("sender_monitor_id" => sender_monitor_id.to_string(), "outlays" => format!("{:?}", outlays)));
         log::trace!(logger, "Building pending transaction...");
@@ -325,6 +328,7 @@ impl<T: BlockchainConnection + UserTxConnection + 'static, FPR: FogPubkeyResolve
             outlays,
             tombstone_block,
             &self.fog_resolver_factory,
+            opt_memo_builder,
             &mut rng,
             &self.logger,
         )?;
@@ -446,6 +450,7 @@ impl<T: BlockchainConnection + UserTxConnection + 'static, FPR: FogPubkeyResolve
             &outlays,
             tombstone_block,
             &self.fog_resolver_factory,
+            None,
             &mut rng,
             &self.logger,
         )?;
@@ -548,6 +553,7 @@ impl<T: BlockchainConnection + UserTxConnection + 'static, FPR: FogPubkeyResolve
             &outlays,
             tombstone_block,
             &self.fog_resolver_factory,
+            None,
             &mut rng,
             &self.logger,
         )?;
@@ -823,6 +829,8 @@ impl<T: BlockchainConnection + UserTxConnection + 'static, FPR: FogPubkeyResolve
     /// * `destinations` - Outputs of the transaction.
     /// * `tombstone_block` - Tombstone block of the transaciton.
     /// * `fog_pubkey_resolver` - Provides Fog key report, when Fog is enabled.
+    /// * `opt_memo_builder` - Optional memo builder to use instead of the
+    ///   default one (EmptyMemoBuilder).
     /// * `rng` - randomness
     /// * `logger` - Logger
     fn build_tx_proposal(
@@ -836,6 +844,7 @@ impl<T: BlockchainConnection + UserTxConnection + 'static, FPR: FogPubkeyResolve
         destinations: &[Outlay],
         tombstone_block: BlockIndex,
         fog_resolver_factory: &Arc<dyn Fn(&[FogUri]) -> Result<FPR, String> + Send + Sync>,
+        opt_memo_builder: Option<Box<dyn MemoBuilder + 'static + Send + Sync>>,
         rng: &mut (impl RngCore + CryptoRng),
         logger: &Logger,
     ) -> Result<TxProposal, Error> {
@@ -867,18 +876,17 @@ impl<T: BlockchainConnection + UserTxConnection + 'static, FPR: FogPubkeyResolve
             fog_resolver_factory(&fog_uris).map_err(Error::Fog)?
         };
 
-        // TODO: Use RTH memo builder, optionally?
+        // Create tx_builder.
+        // TODO (GH #1522): Use RTH memo builder, optionally?
+        let memo_builder: Box<dyn MemoBuilder + Send + Sync> =
+            opt_memo_builder.unwrap_or_else(|| Box::new(EmptyMemoBuilder::default()));
 
         let fee_amount = Amount::new(fee, token_id);
-
-        // Create tx_builder.
-        let mut tx_builder = TransactionBuilder::new(
-            block_version,
-            fee_amount,
-            fog_resolver,
-            EmptyMemoBuilder::default(),
-        )
-        .map_err(|err| Error::TxBuild(format!("Error cretaing TransactionBuilder: {}", err)))?;
+        let mut tx_builder =
+            TransactionBuilder::new_with_box(block_version, fee_amount, fog_resolver, memo_builder)
+                .map_err(|err| {
+                    Error::TxBuild(format!("Error creating transaction builder: {}", err))
+                })?;
 
         // Unzip each vec of tuples into a tuple of vecs.
         let mut rings_and_proofs: Vec<(Vec<TxOut>, Vec<TxOutMembershipProof>)> = rings