R1:c514d34b9456

Add build step implementation infrastructure and sleep build step.

Summary:
Depends on D7498.

This implements support for a "build step implementation". Build steps have an associated class name (which makes the class in PHP) and a details field, which is serialized JSON (same as PhabricatorRepository).

This also implements a SleepBuildStepImplementation which just pauses the build for a specified period of seconds.

Test Plan:
Inserted a build step with `insert into harbormaster_buildstep (phid…