Stop using mwscript
in favor of mwscript-k8s for cleanup_weighted_tags and the cirrus config dump API for check_indices.
Check indices still is hardcoding the connection info and might have to be generalized once we introduce secrets.
Added very minimal tests for check_indices.
Generalize the extraction of the MW NetworkSession secrets in MWClient so that they're re-usable others.
Depends on !3 (merged)
Bug: T378382