support request generator specification

[petri]

Make it possible to specify a request generator thus:

<opname>:
  request:
    generator: pkg.module:generatorcallable 

Provide support for importing and returning the callable, optionally running it destructively (the callable produces the request on its own) or constructively (the callable only amends the request).

It's the job of the application to hook the generator into the http client, for example.

[petri]

done