Make Runtimes Generic

[paulgb]

• [x] Make the executor configuration generic (#713)
• [x] Turn executor config into an enum (#717)
• [x] Make the executor itself generic
  • [x] Move pruning loop into PlaneDocker (#718)
  • [x] Move readiness check into PlaneDocker (#728)
  • [x] Move Docker construction into PlaneDocker (#719)
  • [x] Rename PlaneDocker to DockerRuntime (#720)
  • [x] Make ContainerId an implementation detail of DockerRuntime (#721)
  • [x] Generalize pull (#722)
  • [x] Implement Runtime trait (#723)
  • [x] Move metrics loop into docker side (#724)
  • [x] Replace DockerRuntime with generic Runtime in the main loop
    • [x] Make BackendManager generic over runtime (#727)
  • [x] Refactor codebase to create a module for runtimes
• [x] Send backend ID as an HTTP header to the backend (so that multiple backends can share a port) (#714)
• [x] Spec out a Drone <> Executor wire protocol (#766)
• [x] Implement an executor that uses the wire protocol over domain sockets (#766, #780)
• [ ] Create a mock executor and use it to add integration tests