[avalanche] Add a getrawproof RPC

Summary:
This RPC does a lookup of the node's proofs from the proofid, and return
the proof data on match.

Depends on D9658.

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/D9659