[avalanche] Refactor helpers to build avaproofs messages

Summary:
This refactor makes build_msg_avaproofs more reusable in other tests which
will be leveraged in a future diff. It also cleans up unnecessary and complex
code in abc_p2p_avalanche_remote_proofs which we should discourage copying.

Test Plan:

ninja check-functional

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17834