webrtc_sendrecv.py: Implement all negotiation modes + bugfixes
Earlier, the example only supported one negotiation mode:
- Browser client is running, gstreamer starts a call and sends offer
Now these three modes are also supported:
- Browser client is running, gstreamer starts a call and sends an offer request
- gstreamer connects and waits for browser client to start a call and send an offer
- gstreamer connects and waits for browser client to start a call and send an offer request
The following features are still missing:
- Data channel support
- TWCC support + stats logging