R1:0470125d9e5f

Add skeleton code for webhooks

Summary: Ref T11330. Adds general support for webhooks. This is still rough and missing a lot of pieces -- and not yet useful for anything -- but can make HTTP requests.

Test Plan: Used `bin/webhook call ...` to complete requests to a test endpoint.

Maniphest Tasks: T11330

Differential Revision: https://secure.phabricator.com/D19045