R1:4ca38612f2b0

Add DoorkeeperExternalObjectQuery, expose more ref/import APIs

Summary:
Ref T2852.

- Broadly, we support "I have a Ref, I need a PHID" well but not "I have a PHID, I need a Ref".
- Add DoorkeeperExternalObjectQuery, and use it to query ExternalObjects.
- Allow external objects to be imported by their internal PHIDs. Basically, if we have an edge pointing at an ExternalObject, we can say "load all the data about this" from just the PHID and have it hit all the same code.
- Allow construction of…