R1:bd99a2b81ea5

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…