Fix bug in OpenAPI docs for pseudonymize endpoints

[skykanin]

Some endpoints show the wrong json return type in the OpenAPI documentation. Specifically they're missing the datadoc and metadata fields.

• Check all endpoints for which ones have incorrect docs
• Derive documentation from data classes that are serialized to JSON so that OpenAPI documentation doesn't go out of sync
  • Use @ApiResponse(content = @Content(schema = @Schema(implementation= DTO.class))) annotation for this
  • This can be done once the file pseudonymization endpoints are removed