[avalanche] Add a getflakyproofs RPC to retrieve the list of flaky proofs

Summary:
This returns the list of proofs added via the setflakyproof RPC.

Depends on D16368.

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/D16369