Expose endpoint to control executors

[morgsmccauley]

Coordinator requires a direct control point to Runner so that it can control the state of the existing Executors.

An endpoint with the following capabilities should be exposed:

• Start an Executor
• Stop an Executor
• List existing Executors

To enable a seamless transition to the Control Plane, we should be able to toggle between endpoint based control, and the existing solution in which Runner starts Executors on its own.

[darunrs]

Toggling between endpoint and existing has been moved to this Issue.