Minting logic for votes

[amesgen]

Regularly (in every round, proposed to be ~90s), stake pools need to vote on a recent block (the "pre-agreed block"). We should describe this logic. This will have to be somewhat vague as the details depend on the underlying cryptographic certificate/voting scheme.

Even though we are only tasked with designing an implementation for the pre-alpha version of Peras, it seems sufficiently simple to parameterize this component over the pre-agreement functionality.