Enable cmdlets for Windows PowerShell - #3951

• Enable the following cmdlets for Windows PowerShell on user context.
  • Find-WinGetPackage
  • Get-WinGetPackage
  • Install-WinGetPackage
  • Uninstall-WinGetPackage
  • Update-WinGetPackage
  • Get-WinGetSource
• Move WinGetAssemblyLoadContext.cs as common code between the modules.
• Implement AppDomain handler for the current AppDomain for module's dependencies. However, dependency conflicts can still occur. In order to fix it, one should implement a custom app domain and deal with the serialization boundaries.
• WinRTHelpers is a static class to handle all native calls. Since AppDomain doesn't have something similar to LoadUnmanagedDll, a call to SetDllDirectoryW is needed before calling the native method.

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