Using RTP header extensions is currently not convenient. Users have to handle signals from the RTP payloader and instantiate the extension element themselves, making it impossible to use with gst-launch. Adding a property allowing the payloader to automatically try creating extensions. This should help simple use cases and testing using gst-launch. Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/1022>