[Feature] continuous visualMap to support different color space and custom stop values

### What problem does this feature solve?

Currently the continuous visual map allows only interpolation in an RGBA-Space.
For enabling the advantages of different color spaces (in particular of the [CIELab color space](https://en.wikipedia.org/wiki/CIELAB_color_space) which is better suited to create human-perceptually uniform gradients) it would be best to be able to choose the color space in which the interpolation between two colors is performed.

Currently the continuous visual map allows only equidistant interpolation by using multiple colors in the `inRange` property.
But it would be nice to be able to set such interpolation stop values manually.

### What does the proposed API look like?

` {
visualMap: {
type: "continuous",
colorSpace: "lab" | "rgba" | ...,
inRange: {
stopValues: number[],
colors: string[]
}
}`