[avalanche] Clarify the proof accessor API

Summary:
The getProof() and exists() methods only target valid proof and not the orphans. This diff changes this API so it is easier to retrieve any known proof from its ID.

Ref T1854.

Test Plan:

ninja check-extended

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10370