Add "url_for" utilities to help users build workflows that involve API calls

[jlowin]

Similar to url_for in Flask, that automatically generates the appropriate URL. In a given flow_run, we should have all the information we need to build an API call on the fly. This would make #88 easier.

[lauralorenz]

@jlowin can you expand some more on what the "appropriate URL" is a URL of?

[cicdw]

Closing due to lack of clarity / lack of clear problem this solves.