Refactor: use CloudConnector::AvailableServices

What does this MR do and why?

• Duo workflow should not use ::Gitlab::CloudConnector::SelfIssuedToken because that is backend Cloud Connector code that is invoked by CloudConnector::AvailableServices
• Resolves #485857 (closed)
• This also required setting up the unit primitive in the access_data.yml so this MR partially addresses gitlab-org/duo-workflow/duo-workflow-service#17 (closed)

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Before	After

How to set up and validate locally

I am not going to recommend setting this up locally to test end-to-end because that is complex and Duo Workflow is currently behind a feature flag and used by a few internal people only.

Instead, I will have you validate that the changed code returns a JWT with the expected fields:

• Set up GDK to use self-signed tokens (so that accces_data.yml is loaded) by setting CLOUD_CONNECTOR_SELF_SIGN_TOKENS env var. Docs on that here.
• Open gdk rails c (so that ENV vars are loaded) and check out the JWT generated by: CloudConnector::AvailableServices.find_by_name(:duo_workflow).access_token
• To decode the generated JWT, you can paste it into https://jwt.io/. The JWT should have the "duo_workflow_execute_workflow" and "duo_workflow_generate_token" scopes

Related to #485857 (closed)