[avalanche] Add a peer availability score that aggregates node availability scores

Summary: A peer availability score lets us know at a high level if a peer is serving avalanche traffic. At a minimum this is useful for debugging but new possibilities become available such as staking rewards based on a combination of proof score (stake amount) and peer availability (utility to the network). This patch implements calculating the score but does not use it yet. These scores will be available via RPC in a follow up patch.

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13073