Support export engine extensions and implement an extension for custom fields

Summary:
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.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13049

Differential Revision: https://secure.phabricator.com/D18954