id_str from firestore

Hi, super nice project demonstrating the use of dataclasses in firestore!

I'm new to both frameworks, but I was wondering why you are generating a custom random id (https://gitlab.com/futureprojects/firestore-model/blob/master/firestore_model/init.py#L139) instead of saving the one which is automatically generated by firestore, avoiding potential collisions in your random id generation.

Thanks :)