[avalanche] Add stake contender polling

Summary:
This patch allows stake contenders to be polled (if -avalanchestakingpreconsensus is set). They are not yet reconciled, so no avalanche voting is taking place. Manually set winners polled for will respond as accepted.

Depends on D17144

Test Plan:

ninja check-functional

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17121