Implement `onValuesChanged` prop/callback

[sergei-maertens]

The SDK/parent component must be able to react to value changes inside the form (because of user input) so that backend logic calls can be scheduled (see #78). Ideally, this would be via a prop onValuesChanged={newValues => {...}}.

I'm not sure yet at the moment if we only need to react to value changes because of user input (could be wired up through Formik's handleChange) or to all changes (in which case we'd probably need to run a useEffect hook).

Tasks

• Figure out which events should trigger the callback
• Add the onValuesChanged prop
• Implement the callback behaviour