feat: Added unique_error_count, unique_warning_count, unique_info_count.

[jcpitre]

Summary:
closes #439
Added unique_error_count, unique_warning_count, unique_info_count.

Expected behavior:

The validation report sections of dataset API response should now have unique_error_count, unique_warning_count, unique_info_count.

Testing tips:
api/tests/test_data/test_datasets.json was modified to add more error notices.
You can test using this file (See https://github.com/MobilityData/mobility-feed-api?tab=readme-ov-file#installation--usage). You can modify this file to test different scenarios.

Testers are invited to follow the tips AND to try anything they deem relevant outside the bounds of the testing tips.

Please make sure these boxes are checked before submitting your pull request - thanks!

• Run the unit tests with ./scripts/api-tests.sh to make sure you didn't break anything
• Add or update any needed documentation to the repo
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• Linked all relevant issues
• Include screenshot(s) showing how this pull request works and fixes the issue(s)

[davidgamez]

LGTM!