Add a --conduit-token parameter to arc

Summary: Ref T5955. This makes it easier to write scripts which call Conduit via arc call-conduit.

Test Plan: Used arc --conduit-token ... call-conduit user.whoami to make calls as various users.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: mbishopim3, epriestley

Maniphest Tasks: T5955

Differential Revision: https://secure.phabricator.com/D12717