Flex: Update missing_trip_edge rule to create exception

[emmambd]

Describe the problem

In order to support Flex as a part of https://github.com/MobilityData/gtfs-validator/issues/1721, we need to create an exception for the missing_trip_edge rule.

Depends on #1749

Right now, this error does not trigger on Flex feeds. We need to investigate the bug to understand why.

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