Implement the Query and DocumentReference toString methods

## What feature would you like to see?

Would it be possible to override the default Object/Any toString method with ones that let us see the classes inner contents for easier debugging.

### Proposed Methods
- com.google.firebase.firestore.Query.toString -> delegates to com.google.firebase.firestore.core.Query
- com.google.firebase.firestore.DocumentReference.toString -> delegates to com.google.firebase.firestore.DocumentReference.getPath

In Firebase [DatabaseReference](https://github.com/firebase/firebase-android-sdk/blob/main/firebase-database/src/main/java/com/google/firebase/database/DatabaseReference.java#L558) this is already the current behaviour, but not in Firestore.

## How would you use it?

Help debug issues when you do hundreds of db calls and only a few are failing with `com.google.firebase.firestore.CollectionReference@f72b2a8a` as the "causing query".