Rest Api "GET /states" call

**Describe the problem you have/What new integration you would like**

A new Rest API call "GET /states" that enumerates over all current states and returns the result as a JSON array (or a JSON object with "id" as keys) whould be very useful.

I could be just a variant of "/events" that returns all states in one json datastructure and doesn't keep the connection open when finished.

**Please describe your use case for this integration and alternatives you've tried:**

Two use cases would be:

- easy monitoring: monitoring solutions like Zabbix could poll the device state with one call and use automatic sensor discovery. Currently monitoring different states on one device requires multiple calls and the sensor IDs have to be hardcoded in the monitoring system.

- quick console overview: when developing new automations, getting the state of a device using something like:
```python:
curl -s http://devname/states | python3 -m json.tool
```
... would be really useful.

Start by reading the existing /events endpoint, then trace the API entry point that handles it. The work is done when GET /states returns all current states in one JSON structure and closes the connection after responding.

Written by the indexing model from the issue text.