Clear Cart functionality (aka delete all items in Cart at once)

[SaadBazaz]

Feature Request:
REST API to delete all items in a cart (or delete the cart itself) e.g.

DELETE /shop/api/cart

Reason:
There is currently no way to do this other than running a loop over each cart item and setting its quantity to 0, which is more of a hack and puts load on the server + is not stable enough.