Support dashed outlines for polygons and other area graphics

[jjspace]

Feature

From the forum

I would like to draw dashed outlines when using PolygonGraphics, WallGraphics, EllipseGraphics, and RectangleGraphics to draw a shape.
However, I found that Cesium JS currently does not supoprt this feature. Does Cesium have any plans or roadmaps to add this feature in future.

Polylines support dashed materials but Polygons and the other mentioned graphics only support setting the outline color and width. I think it's perfectly reasonable to have more feature parity and add options to control the outline material or at least add a dashed option specifically.

@ggetz pointed out it would probably be possible to achieve this through primitives and custom appearances but would require a fair bit of work and shader understanding.

Side note, this is similar to #6694 in terms of parity between polygon outlines and polylines