Updated the setup script to allow the user to use a custom Firebase SDK instead of always downloading it.
Updated the setup script to allow the user to use a custom Firebase SDK instead
of always downloading it.

PiperOrigin-RevId: 148164526

FirebaseExtended/cocos2dx-cpp-sample,FirebaseExtended/cocos2dx-cpp-sample,FirebaseExtended/cocos2dx-cpp-sample,FirebaseExtended/cocos2dx-cpp-sample