Allow adding start and end dates to timeframes.txt

### Describe the problem

The only way to currently indicate that a fare policy would apply on specific dates is to relate the records in `timeframes.txt` to a `service_id` found in either `calendars.txt` or `calendar_dates.txt`. This however could be somewhat problematic as it may necessitate duplication of all associated data with the service_ids to properly encapsulate how the fare policy is applied.

### Use cases

As a transit agency,
I would like to promote my "fare free week" in GTFS Fares v2,
So that customers can see that transit is free this week, YAY!

### Proposed solution

Add columns `start_date` and `end_date` to `timeframes.txt`. These columns would be conditionally forbidden if the `service_id` was present.

### Additional information

Example use case in real life: 

Related to #450.