getPartitions on all queries

**Is your feature request related to a problem? Please describe.**
I've seen that the [`getPartitions` API](https://googleapis.dev/nodejs/firestore/latest/CollectionGroup.html#getPartitions) is only available on collection group queries, but not on normal queries. I was wondering why that is. You can always rewrite to use a `collectionGroup`, but it feels weird and unintuitive as a user of the SDK.

**Describe the solution you'd like**
Have partitions on these queries:

```ts
firestore().collection('collectionA/docB/collectionC').getPartitions();
```

**Describe alternatives you've considered**
Alternative, would be to do this:
```ts
firestore().collection('collectionC').getPartitions();
```

And to check for the path ` collectionA/docB/...` on the client.

**Additional context**
This is probably because of the restrictions that [no filters are supported](https://firebase.google.com/docs/firestore/reference/rest/v1beta1/projects.databases.documents/partitionQuery), right?

And to support others there might be the need for creating an additional Firestore index, which is not supported yet.