[avalanche] Return proofids in addition to payout scripts in getWinners

Summary: This is needed to implement a version of setStakingRewardWinners that doesn't lose proofids of winners as they are finalized by avalanche.

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17711