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`?

Start with the linked GTFS Realtime reference.md passage for TripUpdate.TripProperties.shape_id and review the surrounding schedule_relationship and stop_time_update definitions. Check the existing comments for scope, including whether NEW trips are included. Done means reaching agreement on the semantics and updating the specification wording consistently.

索引モデルが issue の本文から書いたものです。