rtsp-client: Fix url for generating key in media factory

rtsp-client: Fix url for generating key in media factory

The mount point at / can be accessed by both the URL forms rtsp://<IP>:<PORT> and rtsp://<IP>:<PORT>/.
To make media factory generating the same key for both the URL forms, the url sent to gst_rtsp_media_factory_construct() needs to be normalized first.
This commit creates a new GstRTSPUrl as the normalized url to send to gst_rtsp_media_factory_construct().

Fixes:https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/1297

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2681>