qtdemux: parse Opus and dOps as qtdemux nodes and add size checks
This allows checking the nodes conformity and dumping parsed values.
Note: Audio Sample Entry version parsing and offset handling is handled as part
of `FOURCC_soun` common processing and in `qtdemux_parse_node`.
Also, only read `stream_count` and `coupled_count` when
`channel_mapping_family` != 0. See:
https://opus-codec.org/docs/opus_in_isobmff.html#4.3.2
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4891>