[avalanche] verify signature on avahello reception

Summary:
Verify the peer's signature using his delegated pubkey.
Add functional tests for the AvaHello handshake sequence.
Add a missing return at the end of the AvaHello branch in net_processing.cpp

Depends on D9564

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: deadalnix, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9565