add more types to `google.cloud.firestore.BaseDocumentReference`

[tharvik]

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Summary of the feature request

many methods of {Async,}DocumentReference (such as id, path, collections, …) are missing some type information, making my strict type checker complain about using Any (type used when not documented). having theses types in the codebase directly will help ensure that we are correctly using theses references.

Desired code experience

_: int = reference.path

Expected results

lint fails with "Incompatible types in assignment"

API client name and version

No response

Use case

catch errors sooner

Additional context

No response