Public key recovery from BLS scheme signatures

[autholykos]

This is low priority and related to optimizing the 96 bytes in the Payload. By extracting the PK from the BLS scheme signature we optimize for space at the expense of slightly more computation. This is to be discussed with @ureeves.

Another benefit from this change would be the capability to use ECDSA signature. Considering that the keys would remain the same, we could introduce this at the later stage with a migration.