R1:8fd256a1fd69

Implement step for publishing files as fragments to Phragment in Harbormaster

Summary: This adds a build step in Harbormaster for publishing file artifacts as fragments in Phragment.

Test Plan:
Created a build plan with the following steps:

* Lease Host
* Upload Artifact
* Publish Fragment

Ran the build plan against a buildable and saw the fragment get created in Phragment. Ran the plan again and saw the fragment get updated with a new version. Modified the file that got uploaded and ran the…