Added feature Cart and fixed major bugs in backend

[Prasadpoojary]

Hi Team,

I have added one more feature to add book/product to cart, remove cart item and list cart item of particular customer. You can find the endpoints below.

ONE MORE IMPORTANT thing I should tell you, I have seen few classes were not there in repository but it was accessed in this repo, it was throwing Compile time error ( classNotFound ). I have fixed everything as of now. Please make sure you verify everything before you merge. As of now everything works fine. Its happy to contribute over springboot project.
Below classes were missing in current repo, I have added below classes to fix the compile time error.

backend/src/main/java/com/company/demo/security/CustomUserDetails.java backend/src/main/java/com/company/demo/security/JwtRequest.java backend/src/main/java/com/company/demo/security/JwtResponse.java backend/src/main/java/com/company/demo/exceptionhandler/ErrorResponse.java

Add to Cart >>
POST : http://127.0.0.1:8888/api/cart/add

list cart items >>
GET : http://127.0.0.1:8888/api/cart/all/BWCUST17601

Remove cart item >>
GET : http://127.0.0.1:8888/api/cart/remove/BWCART17601

Thanks,
Prasad, Coder

[Prasadpoojary]

Hi Arpan,

Please check and merge PR

Thanks,
Prasad