Custom color maps defined via .cpd files should be treated as custom color maps defined in the config file

[konstntokas]

Is your feature request related to a problem? Please describe.
Custom color maps defined via .cpd files can be only used when linked to a data variable in Styles section of the xcube server config file. The custom color maps defined in the config file in the section CustomColorMaps however are created independently of any data variable and are available in the xcube viewer. It would be nice, if all custom color maps are treated the same way.

Describe the solution you'd like
Move the definition of the custom color maps defined via .cpd to the section CustomColorMaps and add an Identifier to it. Subsequently, this identifier can be used to link it to datasets, like it is done for custom color maps defined in the config file.

This issue is related to #1046