[jss-plugin-value-observable] Do not evaluate observables onUpdate process

__Is your feature request related to a problem? Please describe.__

I have encountered with a misunderstanding of the functionality through the use of observables with `react-jss` that makes less useful when try to work with observables injected by the `data` argument of `useStyles`.

This is a vaguely example of I want to do

```javascript
import React, { useRef } from 'react';
import { createUseStyles } from 'react-jss';

// This won't work because the container rule is wrapped by a function
const useStyles = createUseStyles({
container: ({ getMousePosition }) => ({
width: getMousePosition('left'),
}),
});

// This won't work because the style definition is wrapped by a function
const useStyles = createUseStyles((theme) => {
container: ({ getMousePosition }) => ({
width: getMousePosition('left'),
color: theme.primaryColor,
}),
});

const Button = (children, ...restProps) => {
const buttonRef = useRef(null);
// This function creates an observable using the mouse event of the ref passed by argument
const getMousePosition = useMousePosition(buttonRef);
const classes = useStyles({...restProps, getMousePosition});
return (

)
}
```
__Describe the solution you'd like__
The reason why this doesn't work is because the observable plugin do not evaluate `onUpdate` rule so there's no way to pass a observable through the `useStyles` to handle `dynamicStyles`

I solve the problem just implementing the same treatment of `onProcessRule` into `onUpdate` processing,

https://github.com/cssinjs/jss/blob/69caa415f245696a526621711e38934495c8e268/packages/jss-plugin-rule-value-observable/src/index.js#L39-L53

__Are you willing to implement it?__
I'm not fully familiar with the plugin API, but I can contribute on it, and if there is any suggestion on this feature I'm open to follow recommendations.