Allow Harbormaster HTTP steps to pass credentials
Summary: Fixes T4590. Use the credentials custom field to allow Harbormaster HTTP requests to include usernames/passwords.
Test Plan: Ran a build plan with credentials, verified they were sent to the remote server.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T4590
Differential Revision: https://secure.phabricator.com/D8786