[avalanche] Add a way to get stake contender vote status

Summary:
This adds logic for determining the avalanche vote result based on the contents of the cache.

Note that the loop in the rejection case should not adversely impact performance since manual winners should be empty by default and user action would typically only introduce a handful of additional winners.

Test Plan:

ninja check-avalanche-stakecontendercache_tests

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16837