ColorConfigSelect component to select between fixed color or gradient

[targos]

Behavior:

• Select element to choose between "Fixed" or "Gradient" color
• When "Fixed": just select the color with a ColorPickerDropdown
• When "Gradient":
  • Select the color with a GradientSelect
  • Select the data variable with a MeasurementVariableSelect

Props:

interface ColorConfigSelectProps {
  color: ColorConfig;
  onChange: (color: ColorConfig) => void;
  variables: MeasurementVariableOptions[];
}

We will need to add the variable's id to the ColorConfig type for gradient.

Depends on:

• Create ColorPickerDropdown component #370
• MeasurementVariableSelect component #374

[targos]

This is blocked on react-plot currently not supporting variable colors in line series.