Submit VCs with 2 Endpoints: one for Participants and one for everything else

Currently, we have 3 endpoints for submitting VCs (/participants for Participant, /self-descriptions for ServiceOffering, and /self-description/resource for Resource). This follows the Gaia-X conceptual model, but limits the usability toward custom assets, i.e., VC types (#173). The proposal is to have one endpoint which accepts any credential type and let the schema decide if it is accepted or not. "Participant" VCs should also be accepted, but will only get the additional Keycloak setup if submitted through /participants. As discussed in the XFSC Catalogue Developers' Community Call, this endpoint should be able to:

• Accept any VC type
• Accept multiple VC types in a single VP

Where point 2 aligns with how Gaia-X services are used at the moment, where usually one ServiceOffering is submitted with several referenced Resources in one VP.