Unify `asset_vault` add and remove asset APIs

[PhilippGackstatter]

Following the introduction of AssetComposition, we can unify APIs in $kernel::asset_vault:

• add_fungible_asset and add_non_fungible_asset -> add_asset
• remove_fungible_asset and remove_non_fungible_asset -> remove_asset

The advantage of this is:

• It should reduce code as we can use mostly the same logic for both asset types and dispatch when necessary.
• It prepares for custom assets.