pass postpone through staticGenerationStore (#58229)
Instead of requiring React in `maybePostpone` (which is susceptible to referencing the wrong version of React during build time, such as in the case where the static worker patches fetch within an app-route, which doesn't have an experimental runtime), this provides the postpone API to the `staticGenerationStore`. That way we know the API is available in render which is when we'd expect to postpone.
