Firestore: Reference-field support (e.g. similar to @DBref from mongodb)

**Is your feature request related to a problem? Please describe.**
Currently, there seems to be no way to tag fields as foreign keys / references, although firestore has an explicit "datatype" reference. Instead, all sub-objects CRUD operations are executed by value (e.g. the full object is inserted as a child element). References would be needed for real e.g. many-to-many relationships etc., though.

**Describe the solution you'd like**
A simple annotation like mongodb's [@DBRef](https://docs.spring.io/spring-data/mongodb/docs/current/api/org/springframework/data/mongodb/core/mapping/DBRef.html) would make the possibly easiest-to-use solution.

**Describe alternatives you've considered**
As a workaround, one could implement additional service methods that store a manual foreign _id fields instead of the full objects but this is cumbersome to implement. Another solution could be DTO objects that include referenced objects "by value" with a custom mapper that maps them to manual references internally.

**Additional context**
None.

The issue names no files, tests, or entry points. Start by locating the existing Firestore object-mapping implementation and its tests, then determine how annotations and reference-valued fields are currently handled. Done means a defined annotation-based reference model supports the requested relationship behavior with coverage for the relevant CRUD operations.

Written by the indexing model from the issue text.