[FR] Change the firestore db in firestore.client.

In short: There is currently no way to change the firestore database to write to from default.
See question on stackoverflow: https://stackoverflow.com/questions/78533417/how-to-write-to-a-firestore-db-other-than-default-in-a-python-cloud-function

As far as I understand there is currently no option to pass the databaseId to firestore.client or to the options when initializing the app (initialize_app(cred)). You can pass the databaseURL to the options in initialze_app, however the Url cannot be looked up in the firebase console anymore.

In my opinion, it would make sense to passt database as an argument to firestore.client() much like you would when initializing the Client class in google.cloud.firestore directly.
```python
db = firestore.client(app, database="my-db")
```

Alternatively the databaseId could be passed as an option when initializing the app.
```python
app = initialize_app(cred, options={"databaseId": "my-db")
```

I'd be willing to implement this feature.

Start by reading the firestore.client and initialize_app entry points described in the issue, then compare their options with the Google Cloud Firestore Client interface. Resolve whether databaseId belongs on firestore.client or app initialization, and confirm that a non-default database can be selected without relying on databaseURL.

Written by the indexing model from the issue text.