Add support for `PSDscRunAsCredential` input parameter for class-based resources

Summary of the new feature / enhancement

As I user I need backwards compatibility for the input parameter PSDscRunAsCredential for those class-based resources that supports it or require it. Without this existing configurations need to change have resources are used within the same configuration run, and also some resource might not be able to be used by DSCv3.

If a class-based resource implements [DscResource(RunAsCredential = 'Mandatory')] those cannot be used by the DSCv3 yet. Similar, many class-based resources uses [DscResource(RunAsCredential = 'Optional')] which cannot be used by a user either.

With the current functionality all class-based resources DSCv3 defaults to [DscResource(RunAsCredential = 'NotSupported')] even if a class-based resource implements it or not.

Proposed technical implementation details (optional)

I would like it to be possible to use input parameter PSDscRunAsCredential with class-based resources and DSCv3 switches user context as necessary on an individual resource basis.

The issue names no files, tests, or entry points. Start by tracing how DSCv3 invokes class-based resources and handles the PSDscRunAsCredential parameter; done means supporting Optional and Mandatory RunAsCredential resources while switching user context per resource as required.