support pseudo-polymorphic API endpoints

[domanchi]

Background Context

Sometimes, endpoints have the following structure to them:

/{item_type}/{item_id}

where a different item_type would indicate to the server to check a different item_id. This is "pseudo-polymorphic" because the swagger specifications would be the same; but the behavior is different depending on input.

Issue

In a single run, we'd probably want to check for the many different types of item_type, and run tests on their corresponding item_id. How can we support this? I'd imagine the difficult part of this implementation is modifying the fixture architecture such that it can know which object to generate, depending on the item_type.