Create use case and sequence diagrams for a complex workflow

[jandrieu]

The current diagram is confusing because roles may be performed by the same entity.

Let's come up with a use case that at least has a requirement that a holder provide a VC as evidence in order to get some other VC. That request for evidence (and its response) AND the request for the issuance should use the same workflow service (likely as a issuer workflow and then as a verifier workflow).

We'll socialize the use case, then once we agree on a good example, we'll create a sequence diagram to illustrate the flow.