Allow user-supplied filters in FirestoreTemplate

It would be nice to have method `FirestoreTemplate.findAllDocuments` as protected, so it could be overridden in a custom FirestoreTemplate class.

The use case could be if you need to apply the same filtering condition to all queries, e.g. by attributes 'active', 'deleted', 'tenantId' etc.

At the moment this method is private, so the only way to intervene is to use AspectJ, which does not look like a good solution.