Flex: Add location group stops schema

[jcpitre]

Describe the problem

We want to be able to support Flex as a part of #1721, we need to add a schema for the new file location_group_stops.txt

Proposed solution

• New schema
• Make sure notices are created for foreign key references for location_group_id and stop_id (based on reference document)
• Also make sure the notices are created if required fields are missing

Alternatives you've considered

No response

Additional context

No response