Enum metadata support

[azrogers]

Requires CesiumGS/cesium-native#1085 to be merged before this. This change implements enum support for structural metadata in Unreal. The implementation is fairly simple, the scalar values of enums can be obtained using UCesiumMetadataValueBlueprintLibrary::GetInteger or UCesiumMetadataValueBlueprintLibrary::GetInteger64. One wrinkle is in obtaining metadata values as strings - the names corresponding to enum values can only be obtained with the corresponding CesiumGltf::Enum definition. This means that we can't just call UCesiumMetadataValueBlueprintLibrary::GetString on an enum FCesiumMetadataValue, since the metadata value itself doesn't contain enough information to resolve the name. Because of this, I've elected to only implement enum support for UCesiumPropertyTablePropertyBlueprintLibrary::GetString, as with the property table reference we have enough information to resolve the enum names. It would be nice to have full support for GetString on an enum value, but considering our design in Cesium Native I think this might be the best we can do.