webrtc: Fix Python custom signaller receiving SDP offer
The GstWebRTC API web interface defaults to receiving an SDP offer and generating an answer, but this can be overridden by entering "offer options" before clicking to open the remote stream. The Python webrtcsink-custom-signaller.py example failed in this mode as it was coded to only generate an offer and receive an answer. Fix this by implementing support for receiving an offer and sending an answer.