[avalanche] Add an RPC to manually declare a flaky proof

Summary: This allow to create a manual list of flaky proofs based on observation of the network. If a flaky proof is selected for staking reward, an additional proof will be also be accepted as a winner by the node.

Test Plan:

./test/functional/test_runner.py abc_mining_stakingrewards

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16368