[avalanche] Extract a proof verification facility from the RPC

Summary:
This makes it possible to reuse the verification from other RPCs and
return consistent messages. There is no change in behavior.

Depends on D9649.

Ref T1611.

Test Plan:

./test/functional/test_runner.py abc_rpc_avalancheproof

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1611

Differential Revision: https://reviews.bitcoinabc.org/D9646