XCM: Create general asset weight weighing framework

[gavofyork]

Right now the (multi-)asset weighing code is very boilerplatey - it's complex and must be duplicated and customised for all chains which provide asset operations over XCM. Ideally, the asset operation benchmarks (i.e. the set of functions able to implement XcmWeightInfo<RuntimeCall>::withdraw_asset, ::reserve_asset_deposited &c.) should be wholly constructible from the xcm_executor::Config::AssetTransactor type definition.

To do this, we'll need to be able to build an enum AssetImpl of all different implementations of an XCM asset. We will need to able to identify the AssetImpl value from any given MultiAsset::id value. We will also need to iterate over all variants of this type. Finally we'll need to be able to create a MultiAsset value for each variant.

So types which normally implement TransactAsset could also conventionally implement a new, sibling trait BenchmarkTransactAsset. This would provide the above facilities. This trait could have default-implemented functions where the general benchmarking implementation is actually contained. Alternatively it could be contained in a struct which is generic over a BenchmarkTransactAsset impl.

These benchmarking functions could then be conventionally used for implementing the asset instructions of XcmWeightInfo.