Add workflow step property to indicate whether a domain should be included

[kezike]

VPRs enable the ability to attach challenge to protect the verifier and domain to protect the holder. At the moment, there is a workflow.steps[STEP_NAME].createChallenge property that indicates whether a challenge should be created and included in a VPR, but there is no property to indicate whether a domain should be included in a VPR. I propose that we create one to provide a clear directive to exchange steps on how they should handle this. I propose either workflow.steps[STEP_NAME].includeDomain or workflow.steps[STEP_NAME].useDomain, but open to alternative suggestions.