Ability to cache the form state

### Are you submitting a **bug report** or a **feature request**?

feature request

### What is the expected behavior?

Currently I would like to cache the form state and switch to another state by FormSpy. After that, I can use the cached form state to override the original form state.

I expected to have a props that I can pass the form state that I cached from FormSpy, so that I can override it after that to update the form.

```javascript

// other stuff

```

### What's your environment?

"final-form": "^4.4.0",
"final-form-arrays": "^1.0.4",
"react-final-form": "^3.1.5",
"react-final-form-arrays": "^1.0.4",

### Other information

Start by reviewing the FormSpy and Form APIs described in the issue, along with how React Final Form currently stores and updates form state. Determine whether cached state should be accepted as a Form prop or restored through an existing API. Done means a documented, tested way to save state, switch context, and restore it without ambiguity.

Written by the indexing model from the issue text.