[avalanche] Add conflicting proofs to the vote

Summary:
This puts the conflicting proofs to votes on the avalanche network. It's currently limited to polling for proofs only, and never called outside of the tests.

Ref T1854.

Depends on D10219.

Test Plan: ninja check-avalanche-processor_tests

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Maniphest Tasks: T1854, T1634

Differential Revision: https://reviews.bitcoinabc.org/D10027