Allow directive to get existing resource instances

[SteveL-MSFT]

Summary of the new feature / enhancement

For set and test use cases, it might require using information from an existing instance of a resource. Since we now have directives, it would make sense to have an option to inform the engine to always perform a get regardless of the overall operation being performed for the config. I think something like:

directives:
  getExisting: true

Which would fail if the instance doesn't exist. Open question is whether it makes sense to support an export variant. If so, I would prefer to have an enum for get vs export, but I don't see how you could make use of export since the order and number of items can't be determined to be used meaningfully.

Proposed technical implementation details (optional)

No response

[michaeltlombardi]

Agreed that export is difficult to use given that there's zero guarantees of deterministic return data for that operation.

Regarding the get case, checking my understanding:

1. Regardless of the DSC configuration operation (dsc config *), the engine will invoke the get operation against a resource instance with this directive.
2. If the operation fails, DSC follows its normal failure flow for a resource operation failure.
3. If the operation succeeds but the resource doesn't exist (only determinable for resources that use the _exist canonical property), DSC considers the directive to have failed and begins a failure flow.
4. If the operation succeeds and the resource exists, DSC continues operations normally.
5. Accessing the output for a resource instance using this directive uses the normative output lookup expressions, not a special-cased expression.

Is that correct?

[SteveL-MSFT]

I think this can be a dupe of #1034