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