[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.

Start by tracing the Python Admin SDK realtime database entry points shown in the example, especially db.reference() and update(). Compare the requested server-side increment behavior with the linked Stack Overflow question, then identify the existing database tests; done means update() accepts the increment value and verifies the expected Firebase Realtime Database behavior.

Written by the indexing model from the issue text.