[Runtime] Add custom weights for AssetRegistry

[sea212]

The orml-asset-registry pallet does already include a runtime-benchmarks feature, but no benchmark logic yet. Once the benchmark logic is available, register the benchmark and add custom weights for AssetRegistry.

Rating this as "low" priority, because all dispatchables can only be called by authorized origins, which are the council and the root origin (can only be invoked via referendum). Calls consequently happen extremely rarely and the dispatchables are not very compute heavy.