[flutter_ui] Let us use async method to convert map object to custom object when fetching data using queries.

[adar2378]

Currently, it is not possible to convert the data using async methods in 'fromFirestore'. But it would be great to add it. And map the snapshots using asAsyncMap function.

final usersQuery = FirebaseFirestore.instance.collection('users')
  .orderBy('name')
  .withConverter<User>(
     fromFirestore: (snapshot, _) => User.fromJson(snapshot.data()!),
     toFirestore: (user, _) => user.toJson(),
   );