[LBP] KYC hook for LBPs

[jubeira]

KYC will happen offchain.

The vendor will provide a signature in the userdata which validates that the sender is authorized when needed.
E.g: Tally validates an account, signs a message, and includes it in userdata. KYC hook validates the signature, and let's the tx through if valid / reverts if invalid.

This means that we don't need a mapping with authorized addresses, but we do need to know whether the signature was provided by an allowlisted account (which in turn requires some setup / maintenance).

[jubeira]

See #1604 (comment).