File shapes.txt should be Conditionally Required

### Describe the problem

The field `trips. shape_id` (a foreign ID referencing `shapes.shape_id`) is **Conditionally Required**, with the following condition:

> Required if the trip has a continuous pickup or drop-off behavior defined either in `routes.txt` or in `stop_times.txt`

When that condition is true, `shapes.txt` becomes required as well, because `trips.shape_id` cannot be valid without a corresponding `shapes.shape_id` value.

### Use cases

It is relevant for consumers when planning the database schemas, and for evaluating correctness of the feed.

### Proposed solution

I see two possible paths:

1. Preferred: make `shapes.txt` conditionally required

Mark `shapes.txt` as **Conditionally Required** in the _Presence_ column of the _Dataset Files_ table (GTFS Schedule), with this condition:

> Required if the trip has a continuous pickup or drop-off behavior defined either in `routes.txt` or in `stop_times.txt`.

This matches the existing condition on `trips.shape_id` and makes the transitive requirement explicit.

2. Alternative: remove the reference from `trips.shape_id`

If `shapes.txt` is not intended to become conditionally required under that condition, then `trips.shape_id` should not be a Foreign ID in `trips.txt`. In that case, its Type should be changed to **Text** (similar to `direction_id`), because there would be no guaranteed referenced table.

### Additional information

This issue is related to #459, but it has a different purpose, as that one aims to include `shapes.txt` in the recommended tables.

As mentioned in the discussion of that issue, 85% of the feeds in the Mobility Database (as of May 2024) already have `shapes.txt`. Therefore, it shouldn’t be a significant change for producers to mark it as Conditionally Required.