Added a reference to anonymous cart on customer

It is now possible to identify an anonymous cart by using its id or external key. The previously used anonymousCartId will soon be deprecated.

• [API] Added anonymousCart field on customer authentication.
• [API] Added anonymousCart field on customer authentication in-store.
• [API] Added anonymousCart field on customer creation.
• [API] Added anonymousCart field on customer creation in-store customer.
• [API] Deprecated the anonymousCartId field in the listed endpoints above in favor of anonymousCart field.