Refactor `WPSC_Controller_Cart::add` to use `WPSC_REST_Checkout_Controller` and `rest_do_request`

[jtsternberg]

Currently, there is a lot of duplicated functionality. We should default to putting our effort into the new REST API controllers and use those as the base. We'll likely have to make some modifications in WPSC_Controller_Cart to make sure things work from a back-compatibility standpoint (unless this is all TEV2? @JustinSainton?)

[JustinSainton]

All tev2, no need to be back compat w/ tev1 - and A++to reducing duplication.