R1:046c1b5b828d

Expose "isDraft" and "holdAsDraft" revision properties over Conduit

Summary:
Ref T13195. See PHI861. The "+ Draft" flag is not currently exposed over the API, but seems stable enough to expose.

Also expose the "hold as draft" flag, normally `arc diff --draft`.

Today, you can get "+ Draft" with some other state by:

- abandoning a draft revision ("Abandoned + Draft"); or
- using `arc diff --plan-changes` with an older `arc` version ("Changes Planned + Draft").

Test Plan: Called `differential.