Support Checkstyle as an output format for lint results.

Summary: Fixes T4948. Add a lint renderer which supports outputting the lint results in the Checkstyle XML format.

Test Plan: Ran arc lint --xml and inspected the output.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4948

Differential Revision: https://secure.phabricator.com/D9083