Product update and get API

[wenjie-mo]

User Story

As a Seller, I would like to update my product.
As a Buyer, I would like to see a list of products sold by a seller.

Detailed Description

• A product associated with the current user should be updated if the update request is valid. A corresponding response needs to be returned.
• A list of products (length will be given by frontend) associated with a seller should be provided to frontend upon request.
• Refractor and reuse existing code if possible.
• Feel free to change/add/delete interfaces and parameters if necessary.
• Test new APIs with Postman.

Acceptance Criteria

• Given Product information, then update product and return corresponding respoonsess
• Given Seller id and number of items requested, then return a list of products