Replace uriparser usage with CesiumUtility::Uri

[azrogers]

With uriparser getting replaced with ada in CesiumGS/cesium-native#1072, we need to remove the usage of uriparser in Cesium for Unreal so we can remove it from the dependencies entirely. This change utilizes the CesiumUtility::Uri::uriPathToNativePath and CesiumUtility::Uri::getPath methods to resolve file URIs. As part of it, I needed to wrap includes of CesiumUtility/Uri.h in THIRD_PARTY_INCLUDES_START and _END macros as ada references multiple macros in #if directives, such as ADA_USING_LIBRARY, that aren't necessarily defined. This causes errors on MSVC. The THIRD_PARTY_INCLUDES_ macros ignore errors in the included header files.

[kring]

Thanks @azrogers!