XCM WeightTrader: Swap Fee Asset for Native Asset (#1845)

Implements an XCM executor `WeightTrader`, facilitating fee payments in
any asset that can be exchanged for a native asset.

A few constraints need to be observed:
- `buy_weight` and `refund` operations must be atomic, as another weight
trader implementation might be attempted in case of failure.
- swap credit must be utilized since there isn’t an account to which an
asset of some class can be deposited with a guarantee to meet the
existential deposit requirement. Also, operating with credits enhances
the efficiency of the weight trader -
https://github.com/paritytech/polkadot-sdk/pull/1677

related PRs:
- (depends) https://github.com/paritytech/polkadot-sdk/pull/2031
- (depends) https://github.com/paritytech/polkadot-sdk/pull/1677
- (caused) https://github.com/paritytech/polkadot-sdk/pull/1847
- (caused) https://github.com/paritytech/polkadot-sdk/pull/1876

// DONE: impl `OnUnbalanced` for a `fungible/s` credit
// DONE: make the trader free from a concept of a native currency and
drop few fallible conversions. related issue -
https://github.com/paritytech/polkadot-sdk/issues/1842
// DONE: tests

---------

Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>
Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>

cumulus/parachains/runtimes/assets/asset-hub-westend/src/lib.rs

@@ -295,7 +295,7 @@ impl pallet_assets::Config<PoolAssetsInstance> for Runtime {
 	type BenchmarkHelper = ();
 }
 
-/// Union fungibles implementation for `Assets`` and `ForeignAssets`.
+/// Union fungibles implementation for `Assets` and `ForeignAssets`.
 pub type LocalAndForeignAssets = fungibles::UnionOf<
 	Assets,
 	ForeignAssets,
@@ -307,18 +307,21 @@ pub type LocalAndForeignAssets = fungibles::UnionOf<
 	AccountId,
 >;
 
+/// Union fungibles implementation for [`LocalAndForeignAssets`] and `Balances`.
+pub type NativeAndAssets = fungible::UnionOf<
+	Balances,
+	LocalAndForeignAssets,
+	TargetFromLeft<WestendLocation>,
+	MultiLocation,
+	AccountId,
+>;
+
 impl pallet_asset_conversion::Config for Runtime {
 	type RuntimeEvent = RuntimeEvent;
 	type Balance = Balance;
 	type HigherPrecisionBalance = sp_core::U256;
 	type AssetKind = MultiLocation;
-	type Assets = fungible::UnionOf<
-		Balances,
-		LocalAndForeignAssets,
-		TargetFromLeft<WestendLocation>,
-		Self::AssetKind,
-		Self::AccountId,
-	>;
+	type Assets = NativeAndAssets;
 	type PoolId = (Self::AssetKind, Self::AssetKind);
 	type PoolLocator =
 		pallet_asset_conversion::WithFirstAsset<WestendLocation, AccountId, Self::AssetKind>;