Support JSON Schema `$ref` to resolve linked entities

[eriCCsan]

We should support JSON Schema $ref resolution so linked entity structures can be discovered and shown to requesters.
Current handling does not properly use $ref to traverse schema relationships. Because of that, only directly visible fields/entities are considered, while referenced definitions are missed.

This blocks a key use case: showing complete linked data structures.
Example: a Person entity can be linked to n Biosample entities through references. If $ref is not resolved, this relation is incomplete or invisible.

Example use case

• Linked entities are defined as an array of $ref entries.
• Reference format is #/definitions/<trustdeckId>.
• Linked entities: [{"$ref": "#/definitions/<trustdeckId>"}, ...]
• Example relation: Person -> Biosample (1:n)
• Result should expose that Person is linked to multiple Biosample entities via schema references.

Reference:

https://json-schema.org/understanding-json-schema/structuring#dollarref

[muellerarmin]

TODO: Evaluate what's better / more performant: using the $ref or storing a list of IDs directly in the database?