Ref T920. Ref T13235. This adds a `Future`, similar to `TwilioFuture`, for interacting with Amazon's SNS service. This is mostly just for proof of concept since only the new `PhabricatorNotificationSMSManagementWorkflow` can actually send an SMS.
The config options should arguably become part of the existing mailers or notifications config options, instead of the AWS options, because we already plan on supporting other SMS services like Twilio.