frequencies.txt primary key: add end_time, headway_secs & exact_times?

### Describe the problem

Currently, [`frequencies.txt`]()'s primary key does not include `exact_times`:

> Primary key (`trip_id`, `start_time`)

### Use cases

*Isn't it a perfectly valid use case to have >1 frequencies entries** for the same trip and starting at the same time? Some examples:
- two frequencies-based "windows" a different `end_time` & `headway_secs` – e.g. `trip_id=t1`/`start_time=07:00:00`/`end_time=22:00:00`/`headway_secs=600`, and `trip_id=t1`/`start_time=07:00`/`end_time=09:00`/`headway_secs=300` for additional "supporting" buses during rush hour
- two "windows" with all fields equal except `exact_times`

### Proposed solution

I propose to extend the primary key to the other columns (`end_time`, `headway_secs`, `exact_times`), too.

### Additional information

_No response_