### Description
This PR partially reverts changes introduced in
https://github.com/microsoft/onnxruntime/pull/15643
We make two API return std::string always in UTF-8.
We also move the entry points from OrtApiBase to OrtApi to make them
versioned.
### Motivation and Context
`GetVersionString` always returns x.y.z numbers that are not subject to
internationalization.
`GetBuildInfoString` can hold international chars, but UTF-8 should be
fine to contain those.
We prefix them with u8"" in case the compiler default charset is not
UTF-8.
Furthermore, creating platform dependent APIs is discouraged.
`ORTCHAR_T` is platform dependent and was created for paths only.
On non-unix platforms would still produce `std::string` that can only
contain UTF-8
The API was introduced after the latest release, and can still be
adjusted.
### Description
<!-- Describe your changes. -->
Various fixes to the CSharp setup
- fix warnings
- fix invalid tests
- update test sdk nuget package
- enables testing on linux
- fixes issue with some unit tests not running in CI
- run unit tests in linux pipeline using dotnet
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
Unit tests weren't breaking in CIs for both Windows and Linux builds and
should have been.
### Description
Reduce a number of auxillary objects created to reduce GC pressure.
Eliminate GCHandle type of memory pinning in most of the places.
Improve string marshalling by allocating unmanaged memory that does not
require pinning. Change native methods from `IntPtr` to `byte[]`
(marshalling pinning is more efficient).
Allocate input/output UTF-8 names in unmanaged heap for the lifetime of
InferenceSession. So we do not keep converting them and pinning on every
Run.
Introduce a new native API that allows to allocate and convert/copy
strings directly into a native tensor.
The PR delivers around 50% latency improvements and less GC pauses.
Inspired by: https://github.com/microsoft/onnxruntime/pull/15520
### Motivation and Context
Client experience GC pressure and performance degradation when dealing
with string tensors.
Co-Authored-By: @tannergooding
