rtspsrc: Add a signal to allow outgoing messages to be modified or dropped
rtspsrc: Add a signal to allow outgoing messages to be modified or dropped

This feature allows applications to implement extensions to the RTSP
protocol, such as those defined in the ONVIF Streaming Specification.

https://bugzilla.gnome.org/show_bug.cgi?id=762884

GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good