[avalanche] Register votes on conflicting proofs

Summary:
Register the votes on conflicting proofs and return an list of the updates as consequence. The logic is the same than the block, as is demonstrated by the unit tests being compatible with the ProofOnlyTestingContext.

Ref T1854.

Depends on D10250, D10269 and D10270.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10271