[Slider] Gradients for track

[ArchdukeTim]

Describe the solution you'd like
The ability to have the color of the track gradate between colors. Possibly fully support custom drawables like the thumb, but I personally don't care about that functionality.

Describe alternatives you've considered
There are currently no alternatives

[afohrman]

Cool idea, thanks for the feature request! I'll run this by our designers to see whether this is something they'd like us to build in to the component.