Improve API for StructuredDataReferences and FileReferencs

Example:

GET /collections/{collectionId}/dataObjects/{dataObjectId}/fileReferences/{fileReferenceId}/payload
--> List of shepard files (aka file descriptions). This also works when the container is not accessible

GET /collections/{collectionId}/dataObjects/{dataObjectId}/fileReferences/{fileReferenceId}/payload/{oid}
--> A single file payload as octet-stream. Fails when the container is not accessible

GET /collections/{collectionId}/dataObjects/{dataObjectId}/structureddataReferences/{structureddataReferenceId}/payload
--> List of structuredDataPayload (including the actual payload), but without the actual payload when the container is not accessible

GET /collections/{collectionId}/dataObjects/{dataObjectId}/structureddataReferences/{structureddataReferenceId}/payload/{oid}
--> A single structured data payload. Fails when the container is not accessible

GET /collections/{collectionId}/dataObjects/{dataObjectId}/timeseriesReferences/{timeseriesReferenceId}/payload
--> List of timeseries payload with actual payload or empty list when container is not accessible