Expose Differential auxiliary fields in Conduit

Summary: Similar to D785 for Maniphest, expose auxiliary field values via
Conduit.
Test Plan: Ran revision.getinfo on a revision with aux fields, got them in the
response.
Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, jungejason
Differential Revision: 802