dashdec: Expose bandwidth and representation ID as metadata

The primary goal was making it viable to play YouTube/Vimeo/... videos
using the native demuxer, since mpv currently uses a workaround in form
of Edit Decision Lists (EDL).

Implemented features:

1 Exposing id / bitrate as stream metadata (similar to the HLS demuxer)
2 Support for multiple video and audio streams
3 A few minor parts of the specification that are in use at YouTube

Signed-off-by: Steven Liu <lq@chinaffmpeg.org>
