[avalanche] Return if a proof is bound to a peer in getrawavalancheproof

Summary:
This will be useful for writing functional tests.

Ref T1854.

Test Plan:

./test/functional/test_runner.py abc_rpc_avalancheproof

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10736