[avalanche] Add proof-poll responses

Summary:
Nodes will need to come to consensus on which proofs are currently active for any given stake, so they will need to handle Avalanche polls about Avalanche stake proofs themselves.

This change adds basic responses for proof polls.

Depends on D10686, D10904.

Test Plan:
ninja all check-all

python test/functional/abc_p2p_avalanche_voting_proofs.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10387