pallet-asset-conversion: Decoupling Native Currency Dependancy

[muharem]

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, place in code.

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) pallet-asset-conversion: Swap Credit #1677
• (caused) UnionOf types for merged fungible and fungibles implementations #2033
• (caused) Resolve Credit to Account impls of OnUnbalanced trait #1876