Draft: color: Dummy implementations for color representation and parametric image description

For the Gstreamer draft implementation in gstreamer/gstreamer!6830

This is meant as an experiment to explore which functionality we need to implement for initial GstVideoColorimetry support, sending the values needed for the sRGB, bt601, bt709, bt2020-10, bt2100-pq and bt2100-hlq profiles. Values from the client currently only get printed and for now we always use the default sRGB color profile when creating parametric image descriptions.