`AllOk` runs a set of promises concurrently, and like `TryJoin` waits for them all to succeed or one to fail.
Unlike `TryJoin` it returns a single unified status of the composition, so cannot handle member promises that might return `StatusOr` or the like.
Closes#35304
COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35304 from ctiller:all-review 30f5f809c6
PiperOrigin-RevId: 591031189