Add a "Build with CircleCI" build step

Summary: Ref T9456. Some rough edges and we can't complete the build yet since I haven't written a webhook, but this mostly seems to be working.

Test Plan:

• Ran this build on some stuff.
• Ran a normal HTTP step build to make sure I didn't break that.

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Reviewers: chad

Reviewed By: chad

Subscribers: JustinTulloss, joshma

Maniphest Tasks: T9456

Differential Revision: https://secure.phabricator.com/D14286