Allow VectorTilesRasterOverlay elements to be styled from Blueprints or C++

[azrogers]

This PR builds on the VectorStylingProvider introduced in CesiumGS/cesium-native#1430 to allow elements loaded from the vector tiles tileset to be styled using either a specified Blueprint class or from C++.

Because of the peculiarities of how Blueprint works, all the work done by Blueprint has to happen on the main thread. This is a significant hit to performance with large datasets, so the choice was made to allow the user to also specify a VectorStylingProvider directly. It would be nice to work this in to the Unreal class system, but because UObjects are not thread-safe, the easiest way I could think of to make this work was to just have the user specify a TFunction (the Unreal standard library equivalent to a std::function) that returns a shared_ptr to a VectorStylingProvider.