Add serviceDate to Trip model

[miklcct]

Is your feature request related to a problem? Please describe.
The API doesn't have a distinction between Trip and TripOnDate (which can be modified by real-time updates), which causes problem when linking StopTime back to the actual instance of TripOnDate.

In particular, it is not possible to get the full list of StopTimes for a trip from the stop timetable without making an additional API call or using hacks. Also, the API also doesn't support dynamically changing headsigns by real-time updates as well.

Goal / high level use-case
I want to get that a running trip on a date as a model, with real-time diversions in it. It may be different from the schedule.

Describe the solution you'd like
The exact model will need to be discussed. For example, a serviceDate in the Trip model along with the relevant API may be sufficient.

Describe alternatives you've considered
Make Trip an interface and separate them into ScheduledTrip and TripOnDate models. The former can run on multiple dates while the latter represents one of them.

Additional context

[miklcct]

Unfortunately, the model is result of conflating two different concepts which results in bugs such as #6242.

• A scheduled trip which may run on multiple calendar dates (NeTEx Service Journey)
• A particular instance for a trip on a date, where any properties can be modified by real time updates (NeTEx Dated Service Journey)

In our internal model, we already have TripOnServiceDate, and I think that we should deprecate the dated fields in the GTFS Trip model and expose TripOnServiceDate on the API instead, like how Transmodel works.

@t2gran does SIRI-ET support modifying the Destination Display (GTFS: headsign) or Train Number for a Service Journey in real-time?

[miklcct]

superseded by TripOnServiceDate in #5393