[avalanche] First iteration on Proof

Summary: This is an iteration on what proofs might look like.

Test Plan:

ninja all check-avalanche

Reviewers: #bitcoin_abc, jasonbcox, Fabien

Reviewed By: #bitcoin_abc, jasonbcox, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6871