feat(stop_events): add Model.StopEvent

[runkelcorey]

Part 1 following decision to disaggregate #973

Summary of changes

1. Adds Model.StopEvent with fields to
   • uniquely identify the event; I think that a stop should be unique per start_date, trip_id, route_id, vehicle_id, and stop_sequence regardless of mode
   • filter (direction and revenue)
   • name the event (id)
   • describe the actual arrival and departure time
2. Adds stop_sequence type to Model.Schedule so that Model.StopEvent can reference it and make explicit the relationship between these fields.

What questions should reviewers consider?

1. Are there instances where a rider would experience multiple different stops while VehiclePositions shows the same start_date, trip_id, route_id, vehicle_id, and stop_sequence?
2. Should this model have an id field? I don't expect events will often be looked up directly—instead, I expect it will be queried like predictions is queried—but I like to have a descriptive identifier for reference.