[avalanche] Introduce ProofRef as an alias for std::shared_ptr<Proof>

Summary:
This is just shorter and more readable. There is no change in behavior.

Depends on D10355.

Ref T1854.

Test Plan:

ninja all check-all

Should return a single occurrence (the definition):

git grep "std::shared_ptr<Proof>"

Should return no occurrence:

git grep "std::shared_ptr<avalanche::Proof>"

Reviewers: #bitcoin_abc, PiRK, deadalnix

Reviewed By: #bitcoin_abc, PiRK, deadalnix

Subscribers: deadalnix

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10342