Add wrapper for adding entries to a chainstate's block index candidates

Summary:
This is a partial backport of core#27746
https://github.com/bitcoin/bitcoin/pull/27746/commits/10c05710ce1602d932037f72dc6c4bbc3f6f34ba

Depends on D17440

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17532