Implement get() method from native SDKs for Realtime Database

[puf]

The native SDKs added a new get() method to solve an issue with getting stale data on mobile devices when disk persistence is enabled, and the code uses addListenerForSingleValueEvent/observeSingleEvent. The FlutterFire library wraps these last two methods, but does not yet implement the get() method. For feature parity and to allow user to reliable get the latest data, it would be good if FlutterFire also wrapped get().

For context on the problem, see https://stackoverflow.com/questions/34486417/firebase-offline-capabilities-and-addlistenerforsinglevalueevent/34487195#34487195

For details on the difference between get() and once(), see: https://stackoverflow.com/questions/66072471/what-is-the-difference-between-get-and-addlistenerforsinglevalueevent