Add support adding multi track on WebRTC Publish

[SelimEmre]

Add support adding multi-track on WebRTC Publish

[peterzanetti]

@mekya @SelimEmre Is there any progress on this issue? We having working on a pseudo simulcasting approach for our product by publishing 2 streams on 2 connections, and without the server having any knowledge that these 2 tracks belong to the same user, it makes managing it from the app side very complicated.

What we would like to be able to do is:

• use both multitrack publish and multitrack play
• on publish side send 2 or more video tracks.
• on play side, play only 1 track per user at any given time, and manually switch tracks as needed.