Flex: Add locations.geojson schema

[emmambd]

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 locations.geojson.

Proposed solution

• New schema

• Modify and test duplicate_key. Test with
  duplicate-area-548-locations-geojson.zip. Duplicated area_548 in https://data.trilliumtransit.com/gtfs/islandtransit-wa-us/islandtransit-wa-us--flex-v2-TEST.zip to test this.

• 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 foreign_key_violation doesn't catch a missing trip_id #1752

• Special considerations given this is a geojson file when we only supported txt files before

Alternatives you've considered

No response

Additional context

No response

[qcdyx]

Continues #1758
How to setup gradle dependencies of GeoTools?

1. add the following dependencies into gtfs-validator/core/build.gradle
   implementation 'org.geotools:gt-main:31.2'
   implementation 'org.geotools:gt-shapefile:31.2'
   implementation 'org.geotools:gt-geojson:31.2'
   implementation 'org.geotools:gt-api:31.2'
   implementation 'org.locationtech.jts:jts-core:1.19.0'
2. add OGeo repository into the repositories block of gtfs-validator/build.gradle
   maven {
   url 'https://repo.osgeo.org/repository/release/'
   }
3. run ./gradlew build

[jcpitre]

Tasks:

• Read locations.geojson and extract tabular data (id, etc), provide sample validator and notice
• Add validators and notices minimum necessary to remove false positives Flex: Add location group schema and update stop time schema #1749 Modify stops.txt to be conditionally required  #1797
• Obtain geometry data and make overlap calculations
• Add other validators to fully support Flex