[FR] Add the `increment` method for realtime database

**Is your feature request related to a problem? Please describe.**

Please see this SO [question](https://stackoverflow.com/questions/75486368/how-to-increment-values-in-firebase-realtime-database-python/75486555#75486555) that i have asked.

**Describe the solution you'd like**

I would like to be able to increment a value server side such as :

```
from firebase_admin import db

realtime_db = db.reference(path="/", app=app, url="myurl")
realtime_db.update({
f"chats/{uid}/num": db.increment(1),
})
```

**Describe alternatives you've considered**

The alternative described in the SO question as an answer.