[avalanche] sign avaresponse

Summary:
While we ultimately want to use an encrypted and authenticated tunel for avalanche, such as QUIC, having a way to experiment over TCP before this is in place is useful.

TCP is not authenticated, so we need avalanche to sign all messages to do so.

Depends on D5528 and D5536

Test Plan: Updated integration tests to check the signatures.

Reviewers: #bitcoin_abc, Fabien, jasonbcox, nakihito

Reviewed By: #bitcoin_abc, Fabien, jasonbcox, nakihito

Subscribers: nakihito, jasonbcox, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5537