Request to add required start_date property to GTFS-rt TripDescriptor field

[Zach Pepsin]

Outlined in the GTFS-rt docs here: https://developers.google.com/transit/gtfs-realtime/reference/#message_tripdescriptor

The start_date field in the TripDescriptor identifies which date an update corresponds to, and is also required for scheduled trips (as SEPTA uses) per the docs