Support export engine extensions and implement an extension for custom fields
Depends on D18953. Ref T13049. Allow applications and infrastructure to supplement exportable fields for objects.
Then, implement an extension for custom fields. Only a couple field types (int, string) are supported for now.
Test Plan: Added some custom fields to Users, populated them, exported users. Saw custom fields in the export.
Reviewed By: amckinley
Maniphest Tasks: T13049
Differential Revision: https://secure.phabricator.com/D18954