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

closes https://github.com/paritytech/polkadot-sdk/issues/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](https://github.com/paritytech/polkadot-sdk/pull/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) https://github.com/paritytech/polkadot-sdk/pull/1677
- (caused) https://github.com/paritytech/polkadot-sdk/pull/2033
- (caused) https://github.com/paritytech/polkadot-sdk/pull/1876

---------

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

cumulus/parachains/runtimes/assets/common/src/benchmarks.rs

@@ -0,0 +1,44 @@
+// Copyright (C) Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use cumulus_primitives_core::ParaId;
+use sp_runtime::traits::Get;
+use sp_std::marker::PhantomData;
+use xcm::latest::prelude::*;
+
+/// Creates asset pairs for liquidity pools with `Target` always being the first asset.
+pub struct AssetPairFactory<Target, SelfParaId, PalletId>(
+	PhantomData<(Target, SelfParaId, PalletId)>,
+);
+impl<Target: Get<MultiLocation>, SelfParaId: Get<ParaId>, PalletId: Get<u32>>
+	pallet_asset_conversion::BenchmarkHelper<MultiLocation>
+	for AssetPairFactory<Target, SelfParaId, PalletId>
+{
+	fn create_pair(seed1: u32, seed2: u32) -> (MultiLocation, MultiLocation) {
+		let with_id = MultiLocation::new(
+			1,
+			X3(
+				Parachain(SelfParaId::get().into()),
+				PalletInstance(PalletId::get() as u8),
+				GeneralIndex(seed2.into()),
+			),
+		);
+		if seed1 % 2 == 0 {
+			(with_id, Target::get())
+		} else {
+			(Target::get(), with_id)
+		}
+	}
+}