[FEATURE] Allow an optional workflow_step param to the get workflow steps API.

[dbwiddis]

Is your feature request related to a problem?

Users may want to just get the required inputs for a single workflow step rather than the entire list.

What solution would you like?

Add an optional parameter to the get workflow steps API listing one or more comma-delimeted workflow steps, and return a subset of values for just those.

Do you have any additional context?

This could probably be done at the same time as #395