[avalanche] Return the staking reward winner proofid in getstakingreward

Summary: This is possible to have several proofs paying to the same payout, so let getstakingreward discriminate between those.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16367