Implement subcollection support in Firestore

[dmitry-s]

User request: https://stackoverflow.com/questions/62043573/how-to-fetch-documents-from-subcollection-in-firestore-using-spring-data/

Currently we support embedded collections, but Firestore has one more type of associated collections, called subcollections. That feature allows loading the collection independently from the parent document.

[theHilikus]

Is the only way today to use firestore subcollections to use FirestoreTemplate or is it possible to create a Repository extending FirestoreReactiveRepository that supports subcollection documents?
Looking at the sample code and the PR, it seems the answer is the former, but i want to confirm if i'm missing something

[dmitry-s]

@theHilikus you are right, currently we only support FirestoreTemplate way. Please create a ticket if you are interested in an alternative way of accessing subcollections, so we can discuss it.

And also, contributions are welcome!

Thanks

[meltsufin]

Partially fixed in #2415. Please re-open if needed.