Add onRehydrate callback or isHydrated variable

Is your feature request related to a problem? Please describe.
I have been using a bespoke solution to this problem with the vanilla persist middleware and love this higher order approach, however, there is no onRehydrate callback so I can't check if hydration has occurred and if not, block rendering or show a loading screen. In your sample app, it briefly shows 0 before updating to the correct value which is undesirable.

Describe the solution you'd like
I would like to be able to somehow check if the store has hydrated yet.

Describe alternatives you've considered
I have though about how to solve this and don't see any concise way to do so.

Additional context
N/A

Start by locating the persist middleware and the sample app referenced in the issue, then trace when persisted state is rehydrated. Define how a hydration callback or state flag should expose that lifecycle and prevent the sample from briefly showing 0; done means the behavior is covered and the loading state can be detected reliably.