[avalanche] Add a sendavalancheproof RPC shell
This adds a new sendavalancheproof RPC which takes a proof, validates
it, adds it to the pool if not known already and announces it via a
proof inventory message.
Sending the inventory message needs some code in the network layer,
which itself cannot be tested without a RPC to trigger it. This is the
reason why this RPC does not send the proof yet. This will be added in a
follow up diff for ease of review.
This diff contains all the tooling and tests to make it very easy to
complete the RPC once done.
Depends on D9648.
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Maniphest Tasks: T1611
Differential Revision: https://reviews.bitcoinabc.org/D9492