[avalanche] Add a comparator for proof shared pointers

Summary:
This will be useful for sorting proofs by score. Extracted from D10027.

Ref T1854.

Test Plan:

ninja check-avalanche-proof_tests

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10219