[EPIC] Vote Extensions

[jackzampolin]

Vote extensions are not ready to be integrated.

Generally, we do have the hooks in place; the user-defined Consensus Strategy chooses which block (or nil) to vote for, so it could provide additional metadata to be used as a vote extension.

Upon block finalization, the metadata and originating validator can be provided to the consensus strategy, so that the driver can take appropriate actions.

We already have a pattern of "annotations" on blocks and proposed blocks, so we may reuse that word "annotation" for votes as well.

Storing vote extensions/annotations has implications on persisted data like the other steps.