Add is_final_step under processing/training/lambda/function step

[Rosie-Chenyr]

Description of changes:
Add is_final_step under processing/training/lambda/function step

Testing done:
Unit test that the value of is_final_step can be transferred into json when defined

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Tests

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[svia3]

I dont see any unit tests using the @step wrapper that hits this section of code. FunctionSteps are created when users use this annotation. Did we test this / plan to test it? My intuition is that this won't work as they have a different critical path.

[Rosie-Chenyr]

The unit test using the @step decorator is in tests/unit/sagemaker/workflow/test_function_step.py, where I used a current case and defined the value of is_final_step as true and test its result after the step is created.