Implement GetState at GumballMachine

[kaiosilveira]

This method is currently only implemented at the TestingGumballMachine for test purposes as I was worried about allowing clients to change the state of the machine if they decided to call methods on the state objects themselves. To fix this, I could implement the Prototype Pattern on each state so I can call Copy() when returning the state to the client. There’s still a problem with the instance of GumballMachine being kept at each state. I can recreate it defaulting to a NoOpGumballMachine or something