Fail build if an unsupported or unknown operating system is specified in TargetFramework

[dsplaisted]

If the TargetFramework is something like net5.0-mycustomos, we should fail the build with an error that we don't know what mycustomos is.

Suggested logic:

• Platform-specific targets (usually in workloads) will set TargetPlatformSupported to true if the TargetPlatformIdentifier is set to something they recognize and support
• Base SDK will generate an error message if TargetPlatformIdentifier is non-empty and TargetPlatformSupported is not true

If a workload supports a given target platform, it should be responsible for generating error messages if the target platform version specified is one it doesn't support (or doesn't support in combination with the target framework version).

[Nirmal4G]

We should be allowing the optional workloads to understand the custom Frameworks and Platforms. That gives us the opportunity to experiment without modifying NuGet.