[avalanche] Add stake contender to AnyVoteItem

Summary: This adds support in the Processor for stake contenders so we can enable polling and voting.

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17615