Add support for file:// URLs

Unity's [UnityWebRequest](https://docs.unity3d.com/ScriptReference/Networking.UnityWebRequest.html) does not support `file://` URLs, meaning that - unlike Cesium for Unreal ([at least on Windows](https://github.com/CesiumGS/cesium-unreal/issues/378)) - it's not possible for users to use them to access local files. It does support raw file paths, but this sometimes breaks in cesium-native when uriparser doesn't consider a local file path to be a valid URL.

So, we should add explicit `file://` URL support to `UnityAssetAccessor`. When it sees these URLs, it can either read the file from disk directly, or it can turn the URL into a regular file path and pass it to `UnityWebRequest`.