### Description
1. use the pool with VS2022
2. upgrade System.Memory to 4.5.5
### Motivation and Context
Solve the build error while using VS2022:
`[Failure] Msbuild failed when processing the file
'D:\a\_work\1\s\csharp\src\Microsoft.ML.OnnxRuntime\Microsoft.ML.OnnxRuntime.csproj'
with message: Method not found: 'System.ReadOnlySpan`1<Char>
Microsoft.IO.Path.GetFileName(System.ReadOnlySpan`1<Char>)'`
Ref:
https://stackoverflow.com/questions/73399777/azure-build-failing-due-to-method-not-found-system-readonlyspan1char-micros
* Add .net6 support to the C# nuget package.
Currently requires jumping through a lot of hoops due to .net 6 only being supported in the preview release of VS 2022.
Build existing targets using msbuild.
Add .net6 targets and build using dotnet.
Create nuget package with combined targets.
A few misc automated changes from VS to spacing and adding a couple of properties.
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
