[avalanche] Move the VoteRecord to its own header
Summary:
This will make it easier to reuse for voting on proofs.
Ref T1634.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Maniphest Tasks: T1634
Differential Revision: https://reviews.bitcoinabc.org/D9878