Flex: modify foreign_key_violation to support locations.geojson

[emmambd]

Describe the problem

We want to be able to support Flex as a part of https://github.com/MobilityData/gtfs-validator/issues/1721, we need to refactor current rules to support locations.geojson.

Proposed solution

Modify and test foreign_key_violation when location_id is defined in stop_times.txt but not in locations.geojson. Test with foreign_key_violation_area_1137.zip to verify.

• Check if there are performance issues locally with the test feeds in the proposal document, to see if there are persistent issues with parsing failing for Flex data based on what was found in #1752

Alternatives you've considered

No response

Additional context

No response