Add training process attributes on RetrieveWorkerRun

Closes #1341 (closed)

I added a new serializer class instead of adding the attributes to the DataImportLightSerializer, or this would also have affected ListDataImports, RetrieveDataImport, UpdateDataImport, PartialUpdateDataImport, ApplyProcessTemplate, StartProcess, CreateExternalRepository, CreateElementsWorkflow, RetryDataImport, and CreateImportTranskribus.