R1:fe294d4034d7

Allow third-party code to extend upstream datasources via EngineExtension

Summary: Depends on D19089. Fixes T13079. This is likely not the final form of this, but creates a defensible extension point.

Test Plan: See T13079 for discussion.

Maniphest Tasks: T13079

Differential Revision: https://secure.phabricator.com/D19090