ProcessingJob does not have StepID

[momonga-ml]

Currently need to set the name manually for StepId.ProcessingJob.value, as it does not show-up in StepID.

see code under Setup Pipeline with the Step Functions SDK for example:
https://github.com/data-science-on-aws/workshop/blob/master/10_pipeline/stepfunctions/01_Create_Pipeline_Train_and_Deploy_Reviews_BERT_TensorFlow.ipynb

[wong-a]

Steps can accept any string for its name as long as it is unique to the workflow. StepId only really exists for the SDK's prebuilt Pipeline classes, which use the same step names all the time.

Neither using StepId enums or a custom strings for step names is wrong per se.

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Related to #136