Add onRehydrate callback or isHydrated variable

[ClayMav]

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

[gormonn]

@ClayMav Hi. What if you set a default value that explicitly indicates that the data has not been loaded yet?
For example null or undefined.