Adds simple rest middleware for modifying OpenAPI schemas

[mikesol]

Adds simple middleware for modifying OpenAPI schemas based on the request. In this (anti-)pattern, middleware registers itself and de-registers itself. This allows clients to have fine-grained, ad hoc control over the OpenAPI schemas from the test. Similar middleware could theoretically be added after the response is generated as well.

[mikesol]

After this, we'll be able to publish an article for how to test using Meeshkan from NodeJS/python