Define concept on how to add Self Descriptions of types that are currently not being supported

GXFS anticipates use cases, in which Self Descriptions of types other than "Service Offering" or "Participant" (e.g., "Resource", or even arbitrary "Assets" not part of the Gaia-X conceptual model) should be uploaded to the Catalogue. Currently, there are two approaches to do this:

1. Model the SD as a subclass of "Service Offering" or "Participant" - This is a "quick hack" and should not be the final solution
2. Model the asset / resource directly in the Service Offering Self-Description it is linked to.

However, it might be useful to be able to upload these to the catalogue as an independent and correctly modelled Self-Description.

Therefore, the next version of the catalogue should be configurable between being strictly limited to accepting Self-Descriptions of the type "Service Offering" or "Participant", vs. accepting other entity types as well.