Save some output into a variable that can be used in subsequent steps

[jaypipes]

Some things may not be known when writing a GDT test. For example, when creating a resource via a REST API, an auto-generated ID would not be known until the result of a POST call is made. Same for a Kubernetes resource with a generated name.

Provide the ability to specify a field in an action's response that should be saved as a variable that can be referenced in subsequent GDT test steps.