[cloud_firestore_odm] Allow pagination in queries.

[dark-chocolate]

Considering this class:

class User {
  User({
    required this.name,
    required this.age,
  });

  final String name;
  final int age;
}
@Collection<User>('users')
final usersRef = UserCollectionReference();

As of now there's no way of paginating data like:

UserQuery query = usersRef.startAtDocument(userDocumentSnapshot); // No `startAtDocument` found

The current workaround is to get the Query reference from UserQuery and perform the operation on it.