[Lens][Color mapping] Palette authoring

[markov00]

As the following step of the color mapping feature in Lens, we foresee the need for an interface to create and edit data visualization color palettes.

The authoring capabilities could be:

• add/edit custom sets of plain colors
• add/edit custom gradient formulas or simulate them by providing an array of precomputed colors
• provide usage preview in charts on both dark/light mode
• provide guidelines/suggestions on color accessibility (color contrast ratio, color sorting/distribution)

A possible idea of how this could look like and where this could leave is here:

[elasticmachine]

Pinging @elastic/kibana-visualizations (Team:Visualizations)