PR Summary

This work is in the context of Minimal PowerShell.
Cross platform DSC code that was in System.Management.Automation.dll is extracted as a PowerShell subsystem (this PR) and moved to a new Microsoft.PowerShell.DscSubsystem.dll to be shipped with PSDesiredStateConfiguration v3 module (PR in PSDesiredStateConfiguration repo).

PR Context

This PR is following guidelines from subsystem architecture document:

• Cross platform DSC code is removed from System.Management.Automation.dll and placed into a new Microsoft.PowerShell.DscSubsystem.dll in PSDesiredStateConfiguration v3 module.
• Subsystem interface ICrossPlatformDsc is defined in SMA and implemented in Microsoft.PowerShell.DscSubsystem.dll
• Subsystem registration happens dynamically with the help of IModuleAssemblyInitializer.OnImport (implemented in Microsoft.PowerShell.DscSubsystem.dll) when PSDesiredStateConfiguration v3 module is loaded.

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
• This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: or [ WIP ] to the beginning of the title (the WIP bot will keep its status check at Pending while the prefix is present) and remove the prefix when the PR is ready.
• Breaking changes
  • None
  • OR
  • Experimental feature(s) needed
    • Experimental feature name(s):
• User-facing changes
  • Not Applicable
  • OR
  • Documentation needed
    • Issue filed:
• Testing - New and feature
  • N/A or can only be tested interactively
  • OR
  • Make sure you've added a new test if existing tests do not effectively test the code changed
• Tooling
  • I have considered the user experience from a tooling perspective and don't believe tooling will be impacted.
  • OR
  • I have considered the user experience from a tooling perspective and opened an issue in the relevant tool repository. This may include:
    • Impact on PowerShell Editor Services which is used in the PowerShell extension for VSCode
      (which runs in a different PS Host).
      • Issue filed:
    • Impact on Completions (both in the console and in editors) - one of PowerShell's most powerful features.
      • Issue filed:
    • Impact on PSScriptAnalyzer (which provides linting & formatting in the editor extensions).
      • Issue filed:
    • Impact on EditorSyntax (which provides syntax highlighting with in VSCode, GitHub, and many other editors).
      • Issue filed: