Support `--what-if` mode for single-resource invocation

[michaeltlombardi]

Summary of the new feature / enhancement

As an infrastructure engineer, I want to be able to invoke a resource's set operation with the --what-if flag to see how the resource would change the system, so that I can use one-off resource invocations more reliably for interactive and exploratory operations.

As a resource developer, I want to test the behavior of my resource in what-if mode for DSC with minimal overhead and unrelated operations, so that I can progressively enhance my test suite quickly and cleanly.

Right now, invoking resources in what-if mode requires creating a configuration document. While having what-if mode is more valuable for configuration documents, it's also useful for singular resource invocations like:

$properties | dsc resource set -r myOrg/myResource --what-if

This would be useful both for interactive/script users and for resource authors, who can then test the what-if messaging and behavior for their resources with less overhead and a simpler output data model.

Proposed technical implementation details (optional)

I propose that we add the w/--what-if flag to the dsc resource set command, reusing (as much as possible) the logic and code for invoking resources in what-if mode used by the dsc config set command.