webrtcsink: bring in signalling code from whipsink as a signaller

Took a crack at implementing a whip signaller for webrtcsink, using largely unmodified code from whipsink. This can't deprecate whipsink altogether yet because webrtcsink doesn't support encoded input streams yet (though I heard @thiblahute was working on that :)), but can still be useful regardless.

I tested against https://github.com/meetecho/simple-whip-server/, seems to work as expected :)

Example usage:

gst-launch-1.0 -e uridecodebin uri=file:///home/meh/Videos/spring-blender.mkv ! videoconvert ! whipwebrtcsink name=ws signaller::whip-endpoint="http://127.0.0.1:7080/whip/endpoint/room1234"

assuming an endpoint is available at this address ofc.

cc @tkanakamalla @arun :)