Enforce proper JSON-LD and adherance to ODRL

[arnoweiss]

Description

As a user of the Policy Hub
I want to be sure that the Policy Hub serves only valid ODRL
so that I can include it in my EDC-Requests without fear.

Acceptance Criteria

• Validation Scheme written (perhaps extracted from DSP?)
• rudimentary schema validation implemented on POST
• perhaps even check that it's valid json-ld

Additional Information

Saw a couple of errors in a demo. As a first measure, they should be eliminated manually but to prevent this long-term, there should be technical schema enforcement.