add initiator.phid parameter to HM builds

Summary:
Fix T9662.

Record who initiated the build, and allow this information as a parameter.

In this implementation, a 're-run' keeps the original initiator, which we maybe not desired?

Test Plan:
Make a HTTP step with initiator.phid, trigger manually, via HM, via ./bin/harbormaster build.
Look at requests made.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Maniphest Tasks: T9662

Differential Revision: https://secure.phabricator.com/D14380