[avalanche] Require that a pubkey is associated with each avalanche peer.

Summary:
This is going to be necessary to sign responses.

Depends on D5673

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5675