[avalanche] Add nodes using a verifiable delegation instead of asserting the public key

Summary:
As per title. This provides more flexibility as to what key the node is actually using, while ensuring it remains connected to the proof.

The RPC assume the simplest possible delegation for now, mainly to not break callsites. This will need to be upgraded in time.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8604