fix: latest dataset filter and move db models to shared folder under shared folder

[davidgamez]

Summary:

This PR moves the database models to the shared folder to be able to share these transformations with the Python functions. This PR also fixes a failing latest filtering in the datasets endpoint.

Expected behavior:

• For all endpoints except gtfs_feeds/<stable_id>/datasets, the API should behave the same as before, with no changes to the previous behaviour .
• For the gtfs_feeds/<stable_id>/datasets endpoint a valid response in returned.

Testing tips:

Latest dataset fix:

• Hit the local API instance with the following request, replacing the feed_stable_id with a local ID.

curl --request GET \
  --url 'http://localhost:8080/v1/gtfs_feeds/<stable_id>/datasets?latest=true'

• You should expect only the latest dataset returned for the specified feed.

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]

Here is the latest dataset filter fix