adaptivedemux2: Refactor stream methods into the stream

adaptivedemux2: Refactor stream methods into the stream

Unlike the legacy elements, GstAdaptiveDemuxStream is a GObject now,
so a bunch of things that were actually stream methods on the
parent demux object can directly become stream methods now.

Move the stream class out to a header of its own.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3314>