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.

changed milestone to %Arkindex 1.3.2

requested review from @babadie

assigned to @erwanrouchet

mentioned in commit 21022aec

merged