[avalanche] Activate automated peer discovery

Summary:
This diff makes the nodes to be added upon reception of a avahello
message. This feature is hidden behind a dedicated flag so one can
choose to only bind manually submitted nodes.

Depends on D9703.

Ref T1634.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK, baby636

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Maniphest Tasks: T1634

Differential Revision: https://reviews.bitcoinabc.org/D9739