[avalanche] Make peer availability score accessible via getavalanchepeerinfo

Summary: Useful for debugging and generally inspecting the health of the network from the perspective of your node.

Test Plan:
Run multiple times to check for flakiness

ninja
for i in {0..20}; do ./test/functional/test_runner.py abc_rpc_getavalanchepeerinfo || break ; done

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13424