generic /self-descriptions endpoint

resolves issue #218, with the following note:

Accept multiple VC types in a single VP

how important this case, how often do we see it in real life? As of now, there is no any problem to process SD with one VP of basic type and other VCs of the same or 'unknown' type. But, if we have one VC of Participant type and another one of ServiceOffering type, and we want to store them as separate set of claims (with different root uri) in graph DB - then it is not so simple. It will require significant changes on the REST API and DB levels (SD will have many CS/VC IDs). So, the question is: do we really want to process SDs which have VCs in it of different base types?

From what I see in CES events: they return VC with different CSs in it, where one CS can be of type LegalParticipant and another of type ServiceOffering. But when FC resolves these CSs it load their VPs via IDs provided in CS and process loaded SDs independently.