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 Refs from ExternalObjects. This makes the "I have a PHID, I need a Ref" easier.

Test Plan:

• Verified Asana links still enrich properly at display time.
• Used in future revision.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2852

Differential Revision: https://secure.phabricator.com/D6294