Flex: Update missing_trip_edge rule to create exception

[emmambd]

Describe the problem

In order to support Flex as a part of #1721, we need to create an exception for the missing_trip_edge rule.

Depends on #1749

Proposed solution

• Add exception for missing_trip_edge rule:

File name	Spec language	Notice to update or add	Validator logic	Notice table columns
stop_times.txt	arrival_time, departure_time Forbidden** when `start_pickup_drop_off_window` or `end_pickup_drop_off_window` are defined.	missing_trip_edge	If a trip has a start_pickup_dropoff_window or end_pickup_drop_off_window, do not trigger this error.

Test with https://data.trilliumtransit.com/gtfs/islandtransit-wa-us/islandtransit-wa-us--flex-v2-TEST.zip to make sure the error is not triggered.

Alternatives you've considered

No response

Additional context

No response

[qcdyx]

Tasks:

• add the logic to make sure this error doesn't get triggered for flex feeds
• add tests
• review acceptance tests