Apply authorization to add cart endpoint.

[odenizddd]

JWT token will containt username. So get the cart id from username and use that cart id add the specified product. In the add cart endpoint feed the username that comes from the JWT token to fetch the cart id. Disregard the username that comes from the body.

[odenizddd]

I wrote a function that fetches the user id given a username and didn't change the rest of the workflow. It seems to be working.