Making PowerShell adapter and class-based resources cross-platform

[anmenaga]

PR Summary

This removes PowerShell adapter's dependency on PSDesiredStateConfiguration module, therefore making class-based PS resources cross-platform.

There is a number of minor follow-up TODOs in the code that will be filed as issues once this base code is reviewed/approved.

Performance impact: tests showed roughly 50% time improvement over classic Get-DscResource cmdlet; e.g.:

Measure-Command { Get-DscClassResourceDefinitions }
TotalSeconds      : 0.8507316

Measure-Command { Get-DscResource }
TotalSeconds      : 2.1735731