[avalanche] Add a way to get pollable contenders from contender cache

Summary: This provides an easy way to determine all of the contenders that should be polled without introducing deduping logic in Processor to account for the local winner(s). getPollableContenders() will be used in a future patch to reconcile stake contenders.

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17479