rtsp server: allow custom authentication
see issue #502 (closed)
enables subclassing RTSPAuth and overriding authenticate/check/generate_authenticate_header methods in RTSPContext to retrieve request and response and get/replace token added RTSPMessage (GstRtspMessage) with methods to add an authentication header to a request / retrieve authentication parameters from a response
note : first time working on gstreamer/glib so please review carefully
Edited by olivier babasse