[avalanche] Send a getavaaddr message to our avalanche outbound peers

Summary:
This is a step toward active peer discovery: request addresses of good avalanche peers to our avalanche outbounds. Another mechanism will send more getavaaddr message later in case we are still missing nodes, but is out of scope for this diff.

Ref T1696.

Depends on D11010.

Test Plan:

ninja check-extended

Reviewers: #bitcoin_abc, tyler-smith

Reviewed By: #bitcoin_abc, tyler-smith

Subscribers: tyler-smith

Maniphest Tasks: T1696

Differential Revision: https://reviews.bitcoinabc.org/D11020