feat(stop_events): add ApiWeb.StopEventView

[runkelcorey]

Part 2 of disaggregating #973

Summary of changes

1. Adds all attributes from Model.StopEvent to ApiWeb.StopEventView
2. Defines relationships for stop, route, trip, and vehicle using out-of-the-box functionality
3. Adds functionality for schedule relationship by widening State.Schedule.schedule_for and State.Schedule.schedule_for_many to accept a Model.StopEvent

A future PR will add the location for StopEvents along with the StopEventController. I think that's also the right place to test for included trip/stop/route/vehicle lookups that don't return results.

How were these changes validated?

1. Wrote tests for ApiWeb.StopEventView

What questions should reviewers consider?

1. Should we define any other attributes to return via include?
2. Should I update the State.ScheduleTest to test a slightly wider type? I think the current tests are sufficient b/c the functionality doesn't really change