ConnectionEventHandler class Null safety

Represents a class to notify various connection event.

This is base class that you can mixin, so that you can register on SendbirdSdk.addConnectionEventHandler. Typically you want to use this class as following:

class MyHandler with ConnectionEventHandler {
  @override
  void onReconnectionStarted() {
     //user logic here when reconnection is started
  }
  ...
}