Define Duo Workflow Unit Primitive

We must define Unit Primitivie across our systems to integrate Duo Workflow with Cloud Connector.
We already do that in Duo Worfklow (current open MR): !12 (merged):

    DUO_WORKFLOW_EXECUTE_WORKFLOW = "duo_workflow_execute_workflow"

Cloud connector team works on consolidating all UP data in https://gitlab.com/gitlab-org/cloud-connector/gitlab-cloud-connector, but for now, we list UPs in:

• Duo Workflow
• AI GW
• GitLab-rails
• CDot

For Duo Workflow, we think that having them in Duo Workflow and GitLab-Rails is enough for MVP of DW because we expect to launch it on .com first.