Is your feature request related to a problem? Please describe.
Allow color picker render prop

Describe the solution you'd like
Allow renderColorPicker prop to pass value.

interface Props {
  onChange: (color: string) => void;
}
type renderColorPicker = (props: Props) => React.Component;

Describe alternatives you've considered
Build our own color picker. For consistency and desire not to import EUI into elastic charts, the above option is preferable.

Additional Context
The logic to store colors in memory is still available in the legend so this would require handling the handshake between these two things.

Related to old issue #23