feat: add created_at timestamp to feed response

[cka-y]

Summary:

Closes #533 by adding the created_at field to the responses of the basic feed, GTFS feed, and GTFS-RT feed.

Expected behavior:

The created_at field should appear in the API responses for the basic feed, GTFS feed, and GTFS-RT feed. The field should contain a timestamp indicating when the feed was created.

Example response:

{
    "id": "mdb-195",
    "data_type": "gtfs",
    "status": "active",
    "created_at": "2024-07-11T15:24:16.631817Z",
   // other existing fields
}

Testing tips:

• Run the API locally to ensure the changes are correctly integrated.
• Verify that the created_at field is present in the responses for the basic feed, GTFS feed, and GTFS-RT feed.
• Check that the created_at field contains a valid timestamp reflecting the creation time of the 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)

[cka-y]

After this pull request is merged and the database changes are deployed to PROD, we need to ensure to run the following script to update the created_at dates in the database:
created_at_update.sql.zip