Refactoring workflow creation code to remove nested step update

[priyaaank]

I have attempted to improve the following things in this commit. Please take a look and share your opinion.

• Ensure that all public structures are in the top part of the file and all private functions and structs are in the bottom part of the file.
• Add documentation that is reasonable for now, to all the public contracts
• Simplify the public contract as much as possible. So in this case, we have a Workflow, PGWorkflow as types, and CreateWorkflow as the method to create a workflow and ToWorkflow and ToPGWorkflow as the utility conversion methods. To the outside world, this makes the contract small and understandable.
• Instead of doing an if/else or switch case, the class has an enricherMap which holds a step type and an anonymous function for it that will be applied.
• Also note that key is not a plain string, but an alias type for a string called stepType. This adds to readability. Since we want to scope visibility, the type is private.
• The value in the map is also an alias type for a function that takes a step, id for step and returns back an updated step. This allows multiple-step types to be added. However, overall we can possibly find a better mechanism to enrich steps and sub-steps.

I think from the code it is evident that sub-steps were added later on as we handle the id assignment for them separately instead of something like recursion. But I'll think and improve this a bit more.

I have not reviewed the tests and will do that next to improve the coverage and style. Please take a look and let's have a discussion. Having the discussion will promote consistency in code as we all will agree on things.

[tejash-jl]

@priyaaank need to resolve the conflicts