feat: add guest carts items endpoint

Author: ahinkle

src/Api/GuestCarts.php

@@ -13,4 +13,14 @@ public function create()
     {
         return $this->post('/guest-carts');
     }
+
+    /**
+     * List items that are assigned to a specified cart.
+     *
+     * @return string
+     */
+    public function items($cartId)
+    {
+        return $this->post('/guest-carts/'.$cartId.'/items');
+    }
 }

tests/Api/GuestCartsTest.php

@@ -25,4 +25,15 @@ public function test_can_call_guest_carts_create()
 
         $this->assertTrue($api->ok());
     }
+
+    public function test_can_call_guest_carts_items()
+    {
+        Http::fake();
+
+        $magento = new Magento();
+
+        $api = $magento->api('guestCarts')->items('foo');
+
+        $this->assertTrue($api->ok());
+    }
 }