Make some SystemPolicy APIs visible but non-op on Unix so that they can be included in PowerShellStandard.Library - #25051

PR Summary

Some SystemPolicy APIs are useful outside of PowerShell, for example, the ThreadJob module depends on them to determine when to block a thread job from running. However, they are not included in the PowerShellStandard.Library package because they are Windows-only API so far. So, it's hard for a module to use it -- it has to invoke those method with reflection.

In order to include them in the PowerShellStandard.Library package, those APIs need to be available on both Windows and Unix platforms, so this PR make them visible but non-op on Unix, so that they can be included in PowerShellStandard.Library the next time we update that package.

class SystemPolicy
{
    public static SystemEnforcementMode GetSystemLockdownPolicy();
    public static SystemEnforcementMode GetLockdownPolicy(string path, SafeHandle handle);
    public static SystemScriptFileEnforcement GetFilePolicyEnforcement(string filePath, FileStream fileStream);
}

[Update] As pointed out in #25051 (comment), the GetFilePolicyEnforcement method is not available in Windows 5.1 on Win10 or Windows Server 2022. So, we probably should only include GetSystemLockdownPolicy and GetLockdownPolicy in PowerShellStandard.Library for the short/middle term. After Windows 10 and Windows Server 2022 reach EOL (or when the method becomes available on them) we can then add the GetFilePolicyEnforcement method to the package.

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