webrtcsink-signalling: add headers support
During the initial signaling server handshake, add an option to transmit HTTP headers, which can be utilized, for instance, for authentication purposes or sticky sessions:
gst-launch-1.0 webrtcsink signaller::uri="ws://127.0.0.1:8443" signaller::headers="headers,foo=bar,cookie=\"session=1234567890; foo=bar\""