[feature]: add asset metadata to unsigned minting group virtual PSBT

[ffranr]

This PR introduces support for asset group key signatures using an external key during asset minting: #1272.

To enhance functionality, the unsigned PSBT generated during the minting process should include additional asset metadata, such as:

• Asset ID
• Asset name
• Units to be minted

This enrichment would allow external signing software to display helpful asset metadata, providing more transparency and usability during the signing process.

We can add arbitrary fields to psbt.Packet.Unknowns.

Shaping TODO

We need additional shaping on the following:

1. Decide on a global key prefix to ensure uniqueness and avoid conflicts.
2. Define the exact fields that need to be included.
3. Determine how to encode the Value for these fields as bytes. Value field encoding needs to be easy for hardware signers to decode.

[ffranr]

We might want to run this by hardware wallet builders, see what they might to have included as metadata.

Do we need a timestamp to aid in record keeping?

[ZZiigguurraatt]

Is it guaranteed that the metadata will be returned back in the signed PSBT? Seems like the signer has some freedom to remove that kind of information from the PSBT?

[guggero]

Yes, according to the PSBT BIP some fields should be removed after adding the signature (or better the final signature, so all signers still see the metadata until the final one constructs the final witness).
But that shouldn't matter, since the metadata is for the signing device itself. Once it's signed, we don't need it anymore.

[ZZiigguurraatt]

Well, I guess I was wondering if the asset ID could be passed along to the final PSBT, then with #1295 the asset ID could be decoded automatically and then the user would not need to pass the assetid when sealing the batch. Since the metadata can be removed from the PSBT once the final signature is added, it seems there is no way to avoid the need for the user to pass the asset ID when sealing the batch?