[avalanche] Separate pubkey from AvalancheState

Summary:
AvalancheState is really a container for various Avalanche statistics rather than the entire state. Decoupling the pubkey from this makes sense. I made it optional to make it clear that not all nodes must have an Avalanche pubkey.

Future patches will refactor and rename AvalancheState.

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12290