[16.0] shopinvader_api_cart: add /new endpoint

[simahawk]

Allows to force creation of carts the same customer.

[lmignon]

@qgroulard What about the merge mechanism with anonymous users?

[shopinvader-git-bot]

This PR has the approved label and has been created more than 5 days ago. It should therefore be ready to merge by a maintainer (or a PSC member if the concerned addon has no declared maintainer). 🤖

[qgroulard]

@qgroulard What about the merge mechanism with anonymous users?

It will merge one of the anonymous carts with one of the carts of the user.

@simahawk More globally, why do you need several carts and how do you expect them to work ?
Some part of shopinvader are not meant to work with multiple carts (at least I know this method (and the anonymous cart)).