Add Util method to fetch inputs from parameters, content, and previous step output

[dbwiddis]

Description

Adds a Util method to fetch required and optional inputs. Priority is given to:

• Output of previous input steps explicitly stated in the template
• The current node's input parameters (from template)
• The current node's input content (from template)
• Any matching key in any previous step output

If the fetched value is of the form ${{ foo.bar }} then it fetches the value of key bar from step foo.

In addition to the parsing method, I've implemented it in some of the existing steps as an example. Some implementations break existing step tests (because missing input exceptions are different) and can be done later; doing them while awaiting review, but merging this earlier will permit it to be used for current step development.

Issues Resolved

Fixes #210

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[joshpalis]

This is a great addition Dan, everything looks so much cleaner now. Approved, but just had one question for clarification