refactor: add CoinsCachePair alias

Summary:
This is a partial backport of core#28280
https://github.com/bitcoin/bitcoin/pull/28280/commits/75f36d241d2a344c5c4ce2c80250bdde91b3295e
Depends on D18612

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18614