Integrate Harbormaster with Buildkite

Summary: Ref T12173. This might need some additional work but the basics seem like they're in good shape.

Test Plan:

• Buildkite is "bring your own hardware", so you need to launch a host to test anything.
• Launched a host in AWS.
• Configured Buildkite to use that host to run builds.
• Added a Buildkite build step to a new Harbormaster build plan.
• Used bin/harbormaster build ... to run the plan.
• Saw buildkite execute builds and report status back to Harbormaster

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Reviewers: chad

Reviewed By: chad

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T12173

Differential Revision: https://secure.phabricator.com/D17270