[avalanche] Add the availability score to the node statistics

Summary:
This can be useful as a monitoring tool, and as a side effect helps making sure the getavaaddr functional test is setup properly. It can be retrieved via the getpeerinfo RPC.

Ref T1709.

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T1709

Differential Revision: https://reviews.bitcoinabc.org/D11166