Enable `ParallelGroup` resource

[SteveL-MSFT]

Summary of the new feature / enhancement

Enable -parallel switch for dsc

Proposed technical implementation details (optional)

No response

[michaeltlombardi]

This is for applying multiple resources in parallel on the same node, correct? I wonder if this implies the need for something like a canParallel attribute that resources can opt into (treating true as safe for parallel, false as error, and not-set as warn-and-prompt or warn-and-try).