A function that returns a valid authentication token to use with the Firestore REST API, such as an OAuth 2.0 bearer token or a Firebase Auth ID token.
This function is called on every outgoing request and the returned token is not cached by this library. If token reuse, caching, or automatic refresh (e.g. when using short‑lived access tokens) is desired, that logic must be implemented inside this function.
Configuration for using a user-managed authentication token