[avalanche] Consistently use Proof::FromHex() for deserializing a proof

Summary:
This deduplicates the proof deserialization code and add error checking
as needed.

Depends on D9525.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9529