[pallet-assets] Callback function trait for asset registration/deregistration

[shunsukew]

When assets are registered or unregistered, some chains need to do additional updates on their storage. Having injectable callback functions by using traits in pallet-assets increases its extensibility.

For example, all EVM chains need to add RevertCode to pallet_evm::AccountCodes when new assets added to pallet-assets (when deregistering, RevertCode removal needed).

let address = Runtime::asset_id_to_address(asset_id);
pallet_evm::AccountCodes::<Runtime>::insert(address, vec![0x60, 0x00, 0x60, 0x00, 0xfd]);

Otherwise, smart contracts cannot access to assets in pallet-assets via precompiles.

Other usecases for having additional logic when assets are registered/deregistered can be considered.
Allowing this is possible by having a trait in pallet's Config.