Add AWS Region support to PhutilAWSFuture.

Summary:
Almost all AWS services endpoints follow this structure: SERVICE_NAME.REGION_ID.amazonaws.com. This revision adds support for setting a region in PhutilAWSFuture.

There are couple of exceptions to above rule - some services do not have region in their endpoint url (DevPay, Route 53, IAM, Import/Export, Security Token Service), some have a single endpoint in one region (OpsWorks, Support), some are funky (S3), but they all can be covered by overriding getHost() method.

Test Plan: No tests added. I acked through three repositories (libphutil, arcanist & phabricator) and couldn't find AWS used anywhere.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8311