Provide $request->getUser() for Conduit API Requests

Summary:
Allow Conduit methods to retrieve the authoritative, logged-in user
identity.

Test Plan:
Ran user.whoami (an authenticated method) and got my info back. Ran
conduit.connect (an unauthenticated method) and the world did not explode.

Reviewed By: tuomaspelkonen
Reviewers: tuomaspelkonen
CC: tuomaspelkonen, epriestley
Differential Revision: 113