[avalanche] Make ProofRef a shared pointer to a const proof

Summary:
This requires an update of the processor factory method.

Ref T1854.

Depends on D10342.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10356