[Place order] Cart tax

[naydav]

Implement "cart tax" according to proposed scheme
https://github.com/magento/architecture/blob/master/design-documents/graph-ql/coverage/CartPrices.graphqls

The scheme could be changed

Functionality should be covered by API-functional tests

Also, need to check the case described in
#411

[rogyar]

Hi @XxXgeoXxX. I'm accidentally unassigned myself. I'm working on that solution alongside the cart grand total. There's might be just one resolver, so no reason to have a few separate PRs.

[naydav]

#441