User-defined color bars

[forman]

Is your feature request related to a problem? Please describe.

This is the backend implementation for xcube-dev/xcube-viewer#334.

Describe the solution you'd like

Custom color bars are still passed using query parameter cmap, but in the case of custom color bars it is a JSON object with the following format: {"name": <name>, "colors": <colors>}, where <name> is a unique name and <colors> is a list pairs [[<v1>,<c1>], [<v2>,<c2>], [<v3>,<c3>], ...] that map a sample value to a "#RRGGBBAA" color.