webrtc: Add Janus video-room example

This Rust crate provides a program able to connect to a Janus instance using
WebSockets and send a live video stream to the videoroom plugin.

Part-of: <gstreamer/gst-examples!15>
28 jobs for !15 with janus-video-room in 43 minutes and 18 seconds (queued for 7 seconds)
merge request