Separated WinPS adapter and tests

[anmenaga]

PR Summary

Having code for both PS6+ and WinPS adapters in same files is becoming too branchy and hard to maintain;
This PR:

1. moves all WinPS adapter code into a separate files: win_psDscAdapter.psd1, win_psDscAdapter.psm1.
2. WinPS adapter tests are moved out to win_powershellgroup.tests.ps1.
3. PS6+ adapter code is cleaned, e.g. only class-based resources support is left
4. Perf optimizations in PS6+ adapter:
   • only load PSDSC module when actually needed (happens only during cache refresh)
   • remove unnecessary Get-Module -ListAvailable call

PR Context