OS constraint implementation

[vlada-shubina]

Problem

#3107 - OS constraint

Checks:

• Added unit tests
• Added #nullable enable to all the modified files ?

[vlada-shubina]

@baronfel @JanKrivanek looking for the feedback here: do we need to have more features here?

• support other OS
• support exact names (Ubuntu, Alpine ...)
• support OS versions/version range?

As of now only Windows, Linux, OSX are supported (mapped to corresponding OSPlatform).

[JanKrivanek]

I admit I have no insight and understanding of Maui use cases and requirements for our constraints feature. But I'm worried whether this implementation can fullfill them

[baronfel]

Agree with the OperatingSystem-based approach. We don't need to handle OS versions, there's no concrete use case for that that I can see. I'd just make sure that case-insensitive comparisons are handled and then this is good to go IMO!

[vlada-shubina]

Opening for final review, now targeting main.

I decided to stay with OSPlatform as it has definition for various OS Platforms, in contrary to OperatingSystem class. Both are supported in .NET Framework and .NET (Core), so no preference here.

[JanKrivanek]

Looks good!