[feature]: Expose Asset Specifier over RPC

[levmi]

We should expose the Asset Specifier over RPC. After discussion in planning, we decided that was the best path forward to help resolve this issue: #1593

[ZZiigguurraatt]

Do you mean asset_id or group_key?

[guggero]

Both. What we call the asset specifier is a struct that can hold both (or either of them), depending on the use case.