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