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