Add support for webrtc

[MayamaTakeshi]

I forked a project that shows how to use pjlib:
https://github.com/MayamaTakeshi/pjwebrtc

This code is old but it might be good for learning purposes:
https://github.com/MayamaTakeshi/iWebRTC/tree/master/WebRTC%20C%20Sample

But probably this is what we need:
https://github.com/MayamaTakeshi/webrtc-cli

[MayamaTakeshi]

Let's see if we can use this:
https://github.com/shiguredo/libdatachannel

[MayamaTakeshi]

I think there is no need for external libs as it seems pjmedia supports this already:
https://docs.pjsip.org/en/latest/api/generated/pjmedia/group/group__PJMEDIA__TRANSPORT__ICE.html