Swapping flag state "backend"

I'm open to swapable storage

Linking some core issues where it has been decided that ::moduleExists is really all we need for feature flags - but I recognise that's using config which you said you didn't want to use.

That's the same reason for this module existing, client has N sites with common code base and needs to make content changes after deploying new version before turning on new features.

Chatted on slack with @penyaskito - it looks like we have a flag manager service for caching (which is now in core for state so is probably redundant) - but we can keep it there and you can swap that out