Define common ID and PHID export fields in SearchEngine

Summary:
Ref T13049. All exportable objects should always have these fields, so make them builtins.

This also sets things up for extensions (like custom fields).

Test Plan: Exported user data, got the same export as before.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13049

Differential Revision: https://secure.phabricator.com/D18951