[avalanche] Add capability to verify proofs

Summary:
Introduce ProofBuilder to build proofs, and notably sign them properly. These Proofs can then be independently verified.

Depends on D6871

Test Plan:

ninja all check-avalanche

Added many tests cases.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6875