docs: public void concatField(String targetField, String concatString, String... sourceFields) {
          TransformationStep step = new TransformationStep();
          step.setTargetField(targetField);
          step.setSourceFields(sourceFields);
          step.setOperationParameter(TransformationConstants.CONCAT_PARAM, concatString);
          step.setOperationName("concat");
          steps.add(step);
      }