Add basic auxiliary field storage for Differential

Summary:
Precursor to building this out to solve T343. This is similar to the Maniphest
fields we landed recently, although I think they're dissimilar enough that it
isn't worth going crazy trying to make them share code, at least for now.

This doesn't really do anything yet, just adds a storage object and a couple of
selector/field indirection classes.

Test Plan: Ran SQL upgrade script, created an aux field.
Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, jungejason
Differential Revision: 798