Add Cause and DayTimePeriod to GTFS-TripModifications

### Describe the problem

There seems to be information being entered by agency staff but missing from the GTFS-TripModifications messages. SimplifyTransit has spoken with agencies about this data, and we'd like that information included in the Trip Updates feed for all Consumers. Specifically:

- The Cause for the Modification
- The days and time periods when the Modification is happening (I'm calling this DayTimePeriod for lack of a better term), if it's not all day during the service_dates range (common with construction-related detours)

### Use cases

This would allow Consumers to understand more clearly how and when to display an alert to riders, as well as clearly and transparently link a Trip Modification to a related Service Alert.

Without being specific, as we are with other agency entries that lead to Trip Modifications data, there's ambiguity that negatively impacts riders.

### Proposed solution

SimplifyTransit isn't a producer of this data, so I'd look to GTFS-TripModifications Producers to recommend the best solution. Here's what I have in mind so far:

1. **Add a TripModifications.Cause for each Modification**, using the same values from ServiceAlerts.Cause for consistency and clarity between the data feeds. Specify that any Service Alerts related to this Trip Modification must use the same Cause value.
2. **Expand TripModifications.service_dates to include days of the week and time ranges** within those days when the Modification is happening (I'm calling this DayTimePeriod for lack of a better term), if it's not all day during the service_dates range (common with construction-related detours).
3. These fields would be Optional, but the spec would **Require** that Producers who give agency staff fields to enter this structured data **must** provide that data publicly in the Trip Updates feed. If a Producer doesn't give agency staff a way to enter that data, they are recommended to create those fields to supply this data.

Please suggest better approaches and solutions, particularly if you're familiar with how agencies create this data with your product, or will in the future.

### Additional information

_No response_