### Description
1. rename OrtValue.FillStringTensorElement to StringTensorSetElementAt .
To the API user I think we're conceptually setting the string at an
offset in the tensor with is roughly equivalent to `List<string> list
... list[index] = "value"`.
2. While working on new inference examples, I noticed that I am still
inclined to use `DenseTensor` for N-D indexing. Added `GetStrides()` and
`GetIndex()` from strides for long dims, so the user can obtain strides
and translate N-D indices into a flat index to operate directly on the
native `OrtValue` buffers. Expose these functions to the user.
3. Make sure we generate docs for C# public static functions.
### Description
Implement Optional Type metadata support in the library.
Implement optional support in C# API along with metadata.
Implement Sequence, Map, Optional test data support
and test execution.
Prune tests and provide more details for failing tests in C# code.
Note, this PR does not enable running onnx test models in C++.
### Motivation and Context
Opset18 optional type support.
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
