✨(backend) add roomkit viewset to start a room without WebRTC join

Introduce a new viewset that lets the roomkit start a room even when
no WebRTC participant has joined yet.

This is a first entry point that will be extended over time with
more actions a roomkit needs to be able to trigger.

Known limitations:

* The responsibility around SIP rules is currently split between
  the telephony feature and the roomkit one. This may need a
  refactor later on to consolidate ownership in a single place.
* The default throttle might be too low for production usage and
  will likely need to be revisited.

CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to
 - ✨(frontend) add configurable documentation menu item
 - ✨(frontend) allow promoting authenticated participants
 - ✨(frontend) introduce an "unauthenticated" participant badge
+- ✨(backend) add roomkit viewset to start a room without WebRTC join
 
 ### Changed