[avalanche] Introduce the StakeCommitment class

Summary:
This makes the stake commitment strongly typed, in preparation for moving away from proof id. There is currently no change in behavior.

Ref T1676.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T1676

Differential Revision: https://reviews.bitcoinabc.org/D10179