We would previously register a whole bunch of encoder/decoder for which the caps
were ... "unknown/unknown".
Add a function to quickly check (without generating caps) whether a given
AVCodecID has a known mapping (which can include the {video|audio}/x-gst-av-*
ones) without generating the caps.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6237>