Add Flutter examples to sep24.mdx (#187)

* Add Flutter examples to sep24.mdx

* fix formatting of sep24.mdx

docs/building-apps/wallet/sep24.mdx

@@ -29,6 +29,14 @@ val sep24 = anchor.sep24()
 let sep24 = await anchor.sep24();
 ```
 
+```dart
+// By providing the domain hosting the stellar.toml file
+final transferService = await TransferServerSEP24Service.fromDomain("place.domain.com");
+
+// Or by providing the service url
+final transferService = TransferServerSEP24Service("http://api.stellar-anchor.org/transfer");
+```
+
 </CodeExample>
 
 First, let's get the information about the anchor's support for [SEP-24]. This request doesn't require authentication, and will return generic info, such as supported currencies, and features supported by the anchor. You can get a full list of returned fields in the [SEP-24 specification](https://github.com/stellar/stellar-protocol/blob/master/ecosystem/sep-0024.md#info).
@@ -47,6 +55,10 @@ const getAnchorServices = async (): Promise<AnchorServiceInfo> => {
 };
 ```
 
+```dart
+SEP24InfoResponse infoResponse = await transferService.info();
+```
+
 </CodeExample>
 
 ## Interactive Flows
@@ -90,6 +102,14 @@ let deposit = await anchor.sep24().deposit({
 });
 ```
 
+```dart
+SEP24DepositRequest request = new SEP24DepositRequest();
+request.assetCode = "USDC";
+request.jwt = jwtToken;
+
+SEP24InteractiveResponse response = await transferService.deposit(request);
+```
+
 </CodeExample>
 
 As a result, you will get an [interactive response](https://github.com/stellar/stellar-protocol/blob/master/ecosystem/sep-0024.md#deposit-and-withdraw-shared-responses) from the anchor.
@@ -108,6 +128,11 @@ let url = deposit.url;
 let id = deposit.id;
 ```
 
+```dart
+String url = response.url;
+String id = response.id;
+```
+
 </CodeExample>
 
 Similarly to the deposit flow, a basic withdrawal flow has the same method signature and repose type:
@@ -129,6 +154,15 @@ let url = withdrawal.url;
 let id = withdrawal.id;
 ```
 
+```dart
+SEP24WithdrawRequest request = new SEP24WithdrawRequest();
+request.assetCode = "USDC";
+request.type = "bank_account";
+request.jwt = jwtToken;
+
+SEP24InteractiveResponse response = await transferService.withdraw(request);
+```
+
 </CodeExample>
 
 ### Providing KYC Info
@@ -163,6 +197,21 @@ let deposit = await anchor.sep24().deposit({
 });
 ```
 
+```dart
+SEP24DepositRequest request = new SEP24DepositRequest();
+request.assetCode = "USDC";
+request.jwt = jwtToken;
+
+StandardKYCFields kycFields = StandardKYCFields();
+kycFields.naturalPersonKYCFields = NaturalPersonKYCFields();
+kycFields.naturalPersonKYCFields!.emailAddress = "[email protected]";
+kycFields.naturalPersonKYCFields!.photoIdFront = await Util.readFile(path);
+
+request.kycFields = kycFields;
+
+SEP24InteractiveResponse response = await transferService.deposit(request);
+```
+
 </CodeExample>
 
 ### Changing Stellar Transfer Account
@@ -195,6 +244,17 @@ const depositDifferentAccount = async (): Promise<InteractiveFlowResponse> => {
 };
 ```
 
+```dart
+SEP24DepositRequest request = new SEP24DepositRequest();
+request.assetCode = "USDC";
+request.account = "G...";
+request.memo = "my memo";
+request.memoType = "text";
+request.jwt = jwtToken;
+
+SEP24InteractiveResponse response = await transferService.deposit(request);
+```
+
 </CodeExample>
 
 Similarly, for a withdrawal, the origin account of the Stellar transaction could be changed:
@@ -215,6 +275,14 @@ const withdrawal = await anchor.sep24().withdraw({
 });
 ```
 
+```dart
+SEP24WithdrawRequest request = new SEP24WithdrawRequest();
+request.account = "G...";
+//...
+
+SEP24InteractiveResponse response = await transferService.withdraw(request);
+```
+
 </CodeExample>
 
 ## Getting Transaction Info
@@ -279,6 +347,24 @@ const transaction = await anchor.sep24().getTransactionBy({
 });
 ```
 
+```dart
+// single transaction
+SEP24TransactionRequest request = SEP24TransactionRequest();
+request.stellarTransactionId = "transaction id";
+request.jwt = jwtToken;
+
+SEP24TransactionResponse response = await transferService.transaction(request);
+SEP24Transaction transaction  = response.transaction;
+
+// multiple transactions
+SEP24TransactionsRequest request = SEP24TransactionsRequest();
+request.assetCode = "ETH";
+request.jwt = jwtToken;
+
+SEP24TransactionsResponse response = await transferService.transactions(request);
+List<SEP24Transaction> transactions = response.transactions;
+```
+
 </CodeExample>
 
 It's also possible to fetch transaction by the asset
@@ -296,6 +382,15 @@ const transactions = await anchor.sep24().getTransactionsForAsset({
 });
 ```
 
+```dart
+SEP24TransactionsRequest request = SEP24TransactionsRequest();
+request.assetCode = "ETH";
+request.jwt = jwtToken;
+
+SEP24TransactionsResponse response = await transferService.transactions(request);
+List<SEP24Transaction> transactions = response.transactions;
+```
+
 </CodeExample>
 
 ## Submitting Withdrawal Transfer