Resolve "Implement Lambda: count_cars"

Closes #16 (closed)

WARNING: For testing make sure to run the setup_db lambda function from #28 (closed) to populate the s3 bucket with images.

Tested by manual invocation in the browser using the following JSON:

{
    "1704067200": "images/street_2/camera_1/2023-12-18_18-36.png",
    "1704067800": "images/street_2/camera_1/2023-12-18_18-37.png",
    "1704068400": "images/street_2/camera_1/2023-12-18_18-38.png"
}