[avalanche] Add count of finalized proofs to getavalancheinfo
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.
ninja check check-functional
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D11729