Create cart package

[aldy505]

Based on business requirements and architecture design.

Create a cart package that does:

• Addition of a product item to each user's shopping cart
• Modification of each items on user's shopping cart (quantity changes, item deletion, variant changes)
• Other service (eventually) can retrieve user's shopping cart, by:
  • Checking whether a user does have a shopping cart
  • Seeing what's inside a user's shopping cart
• Checkout service will publish a message on Kafka (the topic name and schema is not important right now -- it'll be owned by the checkout package), you should also create an rpc that deletes or clear a user's shopping cart by a specific reason (checkout by system, cleared by system because of too long, cleared by user)

The product schema can be acquired from #4