Formalize custom Conduit fields on objects

Summary: Ref T9964. This just adds more structure to application fields, to make it harder to make typos and easier to validate them later.

Test Plan: Viewed APIs, called some APIs, saw good documentation and correct results.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9964

Differential Revision: https://secure.phabricator.com/D14776