Add Xamarin support to the ORT nuget packages.
- Update C# code to support Xamarin builds for iOS and Android
- refactor some things to split out common code
- include iOS and Android ORT native shared library in native nuget package
* Initial checking for CSharp GPU support
* Enabled C# for GPU build
* Update Onnxruntime to Ort
* Add runtime check for cuda dlls for windows
* Update pretrained model test, for models where name!=model.onnx
* lowered tolerance for float checks to pass new models
* ignore extra ._resnet34v2.onnx file in pretrained test