Add get, startAfter, endBefore query methods for Realtime database

[surajtripathy07]

Currently get, startAfter and endBefore methods are only available for Cloud firestore.

The support was added for Realtime Database in Firebase on February 11, 2021 (release-notes)

This is a limitation within the react-native-firebase library and it would be nice to have the functionality soon 🙂