pallet-asset-conversion: Decoupling Native Currency Dependancy (#2031)

closes #1842

Decoupling Pallet from the Concept of Native Currency

Currently, the pallet is intrinsically linked with the concept of native
currency, requiring users to provide implementations of the
`fungible::*` and `fungibles::*` traits to interact with native and non
native assets. This incapsulates some non-related to the pallet
complexity and makes it less adaptable in contexts where the native
currency concept is absent.

With this PR, the dependence on `fungible::*` for liquidity-supplying
assets has been removed. Instead, the native and non-native currencies'
handling is now overseen by a single type that implements the
`fungibles::*` traits. To simplify this integration, types have been
introduced to facilitate the creation of a union between `fungible::*`
and `fungibles::*` implementations, producing a unified `fungibles::*`
type.

One of the reasons driving these changes is the ambition to create a
more user-friendly API for the `SwapCredit` implementation. Given that
it interacts with two distinct credit types from `fungible` and
`fungibles`, a unified type was introduced. Clients now manage potential
conversion failures for those credit types. In certain contexts, it's
vital to guarantee that operations are fail-safe, like in this impl -
[PR](#1845), place in
[code](https://github.com/paritytech/polkadot-sdk/blob/20b85a5fada8f55c98ba831964f5866ffeadf4da/cumulus/primitives/utility/src/lib.rs#L429).

Additional Updates:
- abstracted the pool ID and its account derivation logic via trait
bounds, along with common implementation offerings;
- removed `inc_providers` on a pool creation for the pool account;
- benchmarks:
-- swap complexity is N, not const;
-- removed `From<u128> + Into<u128>` bound from `T::Balance`;
-- removed swap/liquidity/.. amount constants, resolve them dynamically
based on pallet configuration;
-- migrated to v2 API;
- `OnUnbalanced` handler for the pool creation fee, replacing direct
transfers to a specified account ID;
- renamed `MultiAssetId` to `AssetKind` aligning with naming across
frame crates;

related PRs:
- (depends) #1677
- (caused) #2033
- (caused) #1876

---------

Co-authored-by: joe petrowski <[email protected]>
Co-authored-by: Liam Aharon <[email protected]>

cumulus/parachains/integration-tests/emulated/tests/assets/asset-hub-rococo/src/tests/swap.rs

@@ -266,7 +266,7 @@ fn cannot_create_pool_from_pool_assets() {
 				Box::new(asset_native),
 				Box::new(asset_one),
 			),
-			Err(DispatchError::Module(ModuleError{index: _, error: _, message})) => assert_eq!(message, Some("UnsupportedAsset"))
+			Err(DispatchError::Module(ModuleError{index: _, error: _, message})) => assert_eq!(message, Some("Unknown"))
 		);
 	});
 }

cumulus/parachains/integration-tests/emulated/tests/assets/asset-hub-westend/src/tests/swap.rs

@@ -260,7 +260,7 @@ fn cannot_create_pool_from_pool_assets() {
 				Box::new(asset_native),
 				Box::new(asset_one),
 			),
-			Err(DispatchError::Module(ModuleError{index: _, error: _, message})) => assert_eq!(message, Some("UnsupportedAsset"))
+			Err(DispatchError::Module(ModuleError{index: _, error: _, message})) => assert_eq!(message, Some("Unknown"))
 		);
 	});
 }