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.

[msporny]

The group discussed this on the 2025-03-11 telecon:

@dlongley noted that the create challenge is predicated on create challenge HTTP endpoint in the spec. We don't have such an endpoint for creating/using a domain. Don't know how this directive should work. A domain would either be a static part of your template or a variable when you create an exchange. When an issuer/verifier coordinator creates an exchange, on value could be domain, and domain could be filled in template. One thing that's important, allowing for there to be global variables based on exchange IDs referenced inside templates, maybe we should note that in the spec? @kezike noted that there is another issue around variables, give examples as to how such a thing could be configured by a global variable for exchange by the coordinator. This is related to issue #446.

A PR should be raised to add an example to show how a domain variable can be used in a template to support the use case expressed in this issue.