[Pod] Incorporate signal light with FSM

[taesungh]

We'll want the signal light tower to indicate the active state of the pod. The SignalLight abstraction from #23 provides methods to enable (shine green) and disable (shine red) the light tower.

• Add a field and instance for a SignalLight to StateMachine
• When entering the Running state, enable the signal light
• When entering any other state, disable the signal light
  • Perhaps in the future we could consider also having "exit actions" similar to how we have "enter actions"? There aren't too many other states, so don't worry too much about this for now.