Don't use std::variant in FCesiumPropertyTableProperty

[j9liu]

PropertyTablePropertyViewType was a typedef for a large std::variant defined in Cesium Native. It was so large that all of the normalized property versions had to be put in their own std::variant, and the two were then grouped under their own std::variant in Cesium for Unreal. While Visual Studio's compiler was able to handle this well, Clang would experience incredibly slow compilation times and often would not be able to complete CI.

This replaces the std::variant with a std::any. The original property type is stored in the FCesiumMetadataValueType, and the struct can be used to re-find the C++ type to std::any_cast to. This allows Clang + CI to finally compile successfully, and to somewhat reduce the size of our object files.

[cesium-concierge]

Thanks for the pull request @j9liu!

• ✔️ Signed CLA found.

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• Cesium for Unreal Samples works.