Add backend and frontend part of validator support for JSON-LD

For using STH as vocabulary service inside a DPP ecosystem, it needs to be able to check a DPP (template) for compliance to relevant standards (encapsulated in a message model).

DPP's will be communicated through JSON-LD. Therefore, we need to be able to validate JSON-LD messages.

• How do JSON-LD specifications work? Given it encodes an RDF graph, we assume this can be done with SHACL constraints.