Support additional properties in diff output models

[cka-y]

Summary

Update the Pydantic models in src/gtfs_diff/models.py to allow extra fields in the JSON output, matching the upcoming schema change that sets additionalProperties: true.

Context

The GTFS Diff v2 schema is being updated ([MobilityData/gtfs_diff schema issue]) to allow additional properties on objects, enabling tools to attach custom metadata. The engine's Pydantic models currently use the default strict behavior which will strip/reject extra fields.

[cka-y]

Tasks:

• Suggested test: add an additional property to the python models and validate the schema
• Validate if the additionProporties affects the python model
• Update the model