[Cart Operations] Create Empty Cart

[paliarush]

Implement mutation for empty cart creation by the guest/customer user.

Cart ID is expected in the response. For the guest it should be hashed.

AC:

• createEmptyCart mutation is declared. Should accept no arguments and return cart ID of type String. See example of mutation declaration
• For guest (no token provided via Authorization header), the following service should be used \Magento\Quote\Api\GuestCartManagementInterface::createEmptyCart
• For customer (valid customer token is provided via Authorization header), the followigng service should be used \Magento\Quote\Api\CartManagementInterface::createEmptyCartForCustomer
• Changes are covered with web API functional tests. See \Magento\GraphQl\TestModule\GraphQlMutationTest::testMutation as an example.

[naydav]

#166