Add Conditional Formatting to `ObjectColorBy` input

[AlanRynne]

Dependent on #27

Conditional formatting can be enabled, allowing the user to colour an object using gradients instead of the colour palette.

This may require some changes in the way we're fetching the colors, but it's a pretty self contained area of the visual.

More info: https://learn.microsoft.com/en-us/power-bi/developer/visuals/conditional-format