add helper function extract slots to validate

[wochinge]

With the changes of the RulePolicy, form slots are validated in one custom action action_validate_{form_name}. Any slots which extracted by the form action and have to be validated are appended to the tracker as SlotSet events. So whenever we trigger action_validate_{form_name} from the form action we will send a tracker, which has the following events

tracker_events = tracker_events_from_conversation + [ActionExecuted(form_name)] + slot_set_events_for_newly_extracted_slots

We need to make it easier to validate the newly extracted slots (instead of having users to go through the events one by one) we should add a helper:

    class ValidateSlots(Action):
        def name(self) -> Text:
            return "action_validate_my_form"
        def run(
            self, dispatcher: CollectingDispatcher, tracker: Tracker, domain: Dict
        ) -> List[EventType]:
            extracted_slots: Dict[Text, Any] = tracker.get_extracted_slots()
            validation_events = []
            for slot_name, slot_value in extracted_slots:
                # Simply validation which checks if the extracted slot has a value
                if slot_value is not None:
                    validation_events.append(SlotSet(slot_name, slot_value))
                else:
                    # Return a `SlotSet` event with value `None` to indicate that this
                    # slot still needs to be filled.
                    validation_events.append(SlotSet(slot_name, None))
            return validation_events