Allow for easily sending multiple order items

This allows for the sending of multiple items in a single order, by
adding a new `OrderItem` class and moving all of the order item specific
fields from the `Order` to an individual item.

These can then be assigned to an order using the `withOrderItems()`
method as an array of order items.

Fixes #19

tests/Glassboxx/Request/OrderRequestTest.php

@@ -7,6 +7,7 @@ namespace Opdavies\Glassboxx\Tests\Glassboxx\Request;
 use DateTime;
 use Opdavies\Glassboxx\Request\OrderRequest;
 use Opdavies\Glassboxx\Tests\Glassboxx\TestCase;
+use Opdavies\Glassboxx\ValueObject\OrderItemInterface;
 use Symfony\Component\HttpClient\MockHttpClient;
 use Symfony\Contracts\HttpClient\ResponseInterface;
 
@@ -56,6 +57,7 @@ final class OrderRequestTest extends TestCase
 
         $request = (new OrderRequest($client))
             ->forOrder($this->getMockOrder())
+            ->withOrderItems([$this->getMockOrderItem()])
             ->withAuthToken($authTokenRequest->getToken())
             ->withConfig($this->config)
             ->setCreatedDate('2020-06-04 12:00:00');
@@ -63,4 +65,14 @@ final class OrderRequestTest extends TestCase
         // A successful response returns the original body.
         $this->assertSame(json_encode($body), $request->execute());
     }
+
+    private function getMockOrderItem(): OrderItemInterface
+    {
+        $orderItem = $this->getMockBuilder(OrderItemInterface::class)
+            ->getMock();
+        $orderItem->method('getPrice')->willReturn(7.99);
+        $orderItem->method('getSku')->willReturn('this-is-the-first-sku');
+
+        return $orderItem;
+    }
 }