.NET Standard 2.0 support

[itsWindows11]

In order to support a wide range of .NET versions (& UWP as well), this library should support .NET Standard 2.0.

I'm currently working on an OwlCore.Storage wrapper for this library, and this is necessary. Additionally, a conditional package reference could be added for PolySharp to polyfill missing C# features that do not require runtime support.

[tmds]

While it is possible to support netstandard2.0, it represents a significant effort.

UWP is also embracing the modern .NET: https://devblogs.microsoft.com/ifdef-windows/preview-uwp-support-for-dotnet-9-native-aot/. I think this is the way forward to use Tmds.Ssh in UWP projects.

I'm going to leave the issue open so others may up-vote and chime in if they have an interested.