Added method to restart timer to CountdownController

[acyment]

Hi! I don't fully understand why you have both Countdown and CountdownTimer. I used the former because I needed to pause the timer. I added a method to also restart it. Thanks!

[wuweijian1997]

Hello, thank you very much for submitting your PR.
I think this will also restart the timer,Is there any difference between this and your reset?

   countdownController.value = Duration(seconds: 5).inMilliseconds;
   countdownController.start();

[acyment]

Sorry for not answering before. Both alternatives work, but I just found this interface to be a bit more encapsulated, but it's very subjective.