[xcm] Assets: sort after `prepend_with` (#4235)
Adds sorting to the XCM Assets' `prepend_with`, which could modify the order of `AssetId` locations. Relates to: https://github.com/paritytech/polkadot-sdk/pull/4186 (the same fix for `reanchored`) Part of: https://github.com/paritytech/polkadot-sdk/pull/2129