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