[avalanche] Introduce a Proof::ToHex() shortcut

Summary: As per title. There is currently a couple call sites but more will be added. There is no change in behavior (extensively tested via the RPC).

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11366