persist: provide a way to disable the isolated runtime

[teskje]

This PR introduces PersistClientCache::new_for_turmoil, as a way to use persist in turmoil tests. In particular, this disables the isolated runtime. Tasks that would be spawned there are instead spawned in the current runtime. The method is gated behind a new "turmoil" feature.

Motivation

• This PR adds a test feature that has not yet been specified.

Make it possible to configure persist in a way compatible with turmoil.

Tips for reviewer

Usage example in #34110.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.