Support submission of multiple VCs pointing to the same credentialSubject as "merge" rather than "update"

The desirable way in which the Credential Event Service should handle Gaia-X compliance information as well as further domain-specific information is that we will have two VCs.

Assume the case of two credentials VC1 and VC2 having the same credentialSubject.

As the XFSC Catalogue currently indexes Self-Descriptions by the ID of their credentialSubject, VC2 submitted after VC1 would be treated as an update to VC1, whereas the desired behaviour is that VC1 and VC2 should be merged into one credential talking about the same subject.

Still, it should be possible to submit VC3 pointing to the same credentialSubject to make, say, VC1 deprecated. Seems we'll need to take into account the VCs' IDs.

The GXFS-FR Catalogue apparently already supports that; see first comment below.

First, we need to conceptually understand the implications of VC1 ≠ VC2 but pointing to the same credentialSubject, and should get this right in the following sections of our architecture document:

• Identification of Self-Descriptions
• Self Description Store
• Adding a Self Description for an offering