Add enhanced looping capabilities

[frankkilcommins]

The current specification is arguably Turing complete in so far as it supports conditional branching with the ability to go to other steps or workflows depending on those conditions. Recursively looping it theoretically possible while a particular condition holds true.

In the presentation of the draft specification to the OAS TDC, it was mentioned that the ability to have a more verbose looping process (especially based on the output of a previous step/workflow) might be advantageous for various use-cases.

Example:
Say an API call returns a list of flights matching certain criteria, then one might want to be able to loop over those flights and get flight prices for each of those flights before moving on to a further step.

The purpose of this issue is to discuss the need for a more advance looping system as part of the initial specification version and if warranted propose and add the looping ability to the specification