refactor: require self and sentinel parameters for AddFlags

Summary:
No behavior change. Prepares for adding the CoinsCachePairs to
a linked list when flagged.

This is a partial backport of core#28280
https://github.com/bitcoin/bitcoin/pull/28280/commits/8bd3959feaa0e71585bc5e0976f584fb06ee6d14

Depends on D18614

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18615