[avalanche] Add a ProofPool getProof accessor to retrieve a proof from its id

Summary:
This will be used several times in the code, so wrap it into an accessor.

Ref T1854.

Depends on D10522.

Test Plan:

ninja all check-avalanche

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10586