[avalanche] Add getter and setter RPCs to retrieve/set the staking reward winner

Summary:
Get, recompute and set the payout script from the previous block hash.

Depends on D14648.

Test Plan:

./test/functional/test_runner.py abc_mining_stakingrewards

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14649