Initial support for DTX (EchoTest, VideoRoom)

[lminiero]

Small patch to make EchoTest and VideoRoom aware of DTX in negotiation, so that it can be enabled in case. Not tested extensively yet, which will need to be done to see if there can be an impact on recordings or audio quality in rooms. Since this patch mostly touched the SDP negotiation those plugins do, I took advantage of this to also change something else, e.g., allow the VideoRoom to negotiate stereo on subscribers if the publisher offers it.

Feedback welcome!

[JanFellner]

Lol i always thought DTX would have been working.
We mundged the missing ;usedtx=1 into the SDP answer from janus since january this year.

I am currently using it for
chrome: ">=56",
safari: ">=12.1",
// firefox: ">=78", Due to audio issues we currently disable DTX for firefox
edge: ">=79", // first Chromium version
opera: ">=43" // Chromium 56

Have only had issues with firefox as always the beginning of audio was like cut off. The subscribe did not get the first 1second audio from the publisher so i disabled it for firefox. I could not really nail it down where it comes from.

You only touched the SDP handshaking or also affected modules inside janus?

[lminiero]

Yes, I realize munging SDPs did the job: the main purpose of this PR was to try and avoid the need to munge the SDPs sent to clients. Munging is all we do, as there's nothing else we need to do with traffic.

[lminiero]

Merging.