[avalanche] Add a verifyavalancheproof RPC

Summary: As the name suggests, it runs the verification on the avalanche proof.

Test Plan:

./test/functional/test_runner.py abc_rpc_avalancheproof

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9525