Add basic typechecking support to Conduit

Summary:
Ref T9964. I want to show users what we're expecting in "constraints", and let constraints like "authors=epriestley" work to make things easier.

I'm generally very happy with the "HTTPParameterType" stuff from EditEngine, so add a parallel set of "ConduitParameterType" classes. These are a little simpler than the HTTP ones, but have a little more validation logic.

Test Plan:
This is really just a proof of concept; some of these fields are now filled in:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9964

Differential Revision: https://secure.phabricator.com/D14763