Add a Duo API future

Summary: Depends on D20025. Ref T13231. Although I'm not currently planning to actually upstream a Duo MFA provider, it's probably easiest to put most of the support pieces in the upstream until T5055.

Test Plan: Used a test script to make some (mostly trivial) API calls and got valid results back, so I think the parameter signing is correct.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13231

Differential Revision: https://secure.phabricator.com/D20026