[avalanche] introduce LimitedProofId

Summary:
This stores the intermediate hash generated when building the ProofId as
a member of the Proof class, and adds an accessor.

The next step is to send this limited proof id with the avahello message.

Depends on D9543

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9544