Retrieve the route polyline for an estimated route by its ID.

The `estimated_route_id` is provided in `eta_change` and `eta_change_v2` webhooks.
Use this endpoint to fetch the route polyline that was used for the ETA calculation.

The response includes:
- The route polyline as a GeoJSON LineString
- Start and end locations
- Total distance (meters) and duration (seconds) of the route