Hash `AssetVaultKey` before inserting into asset vault SMT

[PhilippGackstatter]

The asset vault key must be hashed before using it as the key in the asset vault SMT (see #2328 (comment)).

We should implement this in the kernel, the AssetVault in Rust and update the Asset docs to mention this detail.

This should only be done once #2437 is merged into next.

[swaploard]

I can work on this issue.

[PhilippGackstatter]

Hi @swaploard, thanks for your interest! After thinking about it some more, this issue is actually not a "good first issue", since it requires updating the AssetWitness to track the original keys, analogously to StorageMapWitness. So, it may be better to work on something else.