Feature Request: NFT proofs

Usability problem is that metamask-mobile has no way to prove via a QR code that an NFT owner is in fact a verifiable owner.
Expected behaviour. Have metamask create a QR code of a signature with the data being the NFT contract address and
the tokenId. That way others can scan the QR code and check on chain (The NFT contract address) wether the signature is valid and that the NFT in question belongs to the signature address. I expect Metamask to come around to this concept eventually. As it is a much needed feature.

No files, tests, or entry points are named. Start by clarifying the signature and QR-code format, the NFT ownership checks, and how another user would verify them on chain; done would mean an agreed end-to-end proof and verification flow for the contract address and tokenId.

Written by the indexing model from the issue text.