Allow build steps to create URI artifacts

Summary: Ref T1049. This allows build steps to create URI artifacts, which can be used to link to external builds and other resources.

Test Plan: Used a build step in an external library to test the creation of a URI artifact and verified it appeared correctly.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D10173