dashsink: Add signals for allowing custom playlist/fragment

dashsink: Add signals for allowing custom playlist/fragment

Instead of always going through the file system API we allow the
application to modify the behaviour. For the playlist itself and
fragments, the application can provide a GOutputStream. In addition the
sink notifies the application whenever a fragment can be deleted.

Following the HLS change:
https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/918

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2064>