[avalanche] implement getavalanchepeerinfo

Summary: This adds a RPC command similar to existing command getpeerinfo, but for avalanche peers. It is useful for debugging.

Test Plan:
Manually add a few avalanche peers, run the RPC and verify the displayed information.

src/bitcoin-cli getavalanchepeerinfo

Reviewers: #bitcoin_abc, deadalnix, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9290