Add return type to conduit.query

Summary: Fixes T6950. Adds the return type of Conduit API methods to the conduit.query call.

Test Plan: Called echo '{}' | arc call-conduit conduit.query and verified that the return types were present in the response.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T6950

Differential Revision: https://secure.phabricator.com/D11466