Allow defining new `Shape` for trips with `TripUpdate.TripDescriptor.schedule_relationship=REPLACEMENT`

### Describe the problem

When using `TripUpdate.TripDescriptor.schedule_relationship=REPLACEMENT`, a new stop sequence must be given in `TripUpdate.stop_time_update`. However, the current wording of `TripUpdate.TripProperties.shape_id` states:

https://github.com/google/transit/blob/474750a163088673df718838d4a1bb093391f9af/gtfs-realtime/spec/en/reference.md?plain=1#L267

> The order of stops (stop sequences) for this trip must remain the same as (CSV) GTFS.

Would it make sense to have the possibility of defining a new shape for trips with changed stop sequences?

### Use cases

A trip has its stop sequence replaced and now the old shape does not fit the new stops. Updating the shape would improve the end user experience.

### Proposed solution

Change the wording from

> The order of stops (stop sequences) for this trip must remain the same as (CSV) GTFS.

to this

> The order of stops (stop sequences) for this trip must remain the same as (CSV) GTFS, unless `schedule_relationship` is `REPLACEMENT`, in which case `shape_id` may instead describe the vehicle path for the new stop sequence declared in `TripUpdate.stop_time_update`.

### Additional information

I could not find a previous issue related to this. Also, should the wording include other types of trips, for example, `TripUpdate.TripDescriptor.schedule_relationship=NEW`?

Commencez par le passage lié de reference.md concernant TripUpdate.TripProperties.shape_id et examinez les définitions environnantes de schedule_relationship et stop_time_update. Vérifiez la portée dans les commentaires existants, notamment si les trajets NEW sont inclus. Le travail est terminé lorsqu’un accord est trouvé sur la sémantique et que le texte de la spécification est mis à jour de manière cohérente.

Rédigé par le modèle d'indexation à partir du texte de l'issue.