Enables subclassing gst_rtsp_server::RTSPAuth and overriding its
authenticate/check/generate_authenticate_header methods
Also add new methods in RTSPContext to retrieve RTSP request/response, and to
get/replace tokens.
Additionally, added RTSPMessage with methods to add an authentication
header to a request / retrieve authentication parameters from a
response.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1359>
