[avalanche] Add count of finalized proofs to getavalancheinfo

Summary:
Track which peers have finalized their proof and provide the total number of finalized proofs via RPC. This is helpful for testing.

Depends on D11701 for functional test improvements.

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D11729