Prototype node API includes new "finality"-specific APIs

[shielded-nate]

These new APIs:

• need to be well documented and have integration / end-to-end tests.
• use the newest RPC/IPC protocol used in upstream, or if there's none newer than Json-rpc, we create a wholly new RPC interface with a newer/better protocol and tooling. (This needs to be an early / ongoing conversation with upstream Zebra devs.)

What are the actual calls? This isn't a requirements/goals list, but a brainstorm:

• notify when new block final.
• notify when this specific block becomes final or is known to never be finalizeable.
• same as previous for specific txn.
• information about finalization protocol - e.g. what are the proposals, what are the votes? Some of these details belong in a semi-distinct "PoS information API".