[avalanche] Add node related functions to the peermanager

Summary:
This adds a way to register nodes to the PeerManager and select one available for query.

The logic is similar to what is found in AvalancheProcessor and is meant to eventually replace it.

Depends on D6818

Test Plan:

ninja all check-avalanche

Due to the inherent randomness of the process, fully unittesting this is not possible and once more things are pieced together, inetgration tests will have to cover it.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6822