mu invoke: support web requests

[rsyring]

It could be helpful to be able to invoke the local docker runtime in a similar way to how the API gateway would invoke it. This could be an option to mu invoke like:

$ mu invoke --wsgi-url /hello-world

Invoke would then need to construct a minimal dictionary patterned after what the WSGI handler expects. An example of that can be found at mu_tests.data.event_wsgi.

But I wouldn't spend a whole lot of time adding extra options to be able to cover all the different ways a web request might need to be constructed. If that needs to be tested, just do a dev deployment and test it with a browser or http cli client. I guess, we could support something like a --event-json=/some/file/path.json like option which could be a WSGI event. But, at that point, it's easier/faster to just add a test for it and skip trying to sending it through the docker runtime.