refactor: Enable proof reward rank to be used elsewhere
Summary:
Staking rewards pre-consensus will need to recompute reward rank. These future call sites should share the same code path as staking rewards.
There is no change in behavior.
Test Plan:
ninja check check-functional
Reviewers: O1 Bitcoin ABC, #bitcoin_abc, PiRK, Fabien
Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, PiRK, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D16694