Added getShippingLabelHeaders to retrieve header info from the shippi…

…ng label endpoint. This contains the track and trace code.

src/Client.php

@@ -1800,6 +1800,31 @@ public function getShippingLabel(string $shippingLabelId): ?string
         return $this->request('GET', $url, $options, $responseTypes);
     }
 
+    /**
+     * Retrieves the headers of a shipping label by shipping label id. Shipping label metadata, for example the X-Track-And-Trace-Code, are added as headers
+     * in the response.
+     *
+     * @param string $shippingLabelId
+     * @return string|null
+     * @throws Exception\ConnectException
+     * @throws Exception\Exception
+     * @throws Exception\RateLimitException
+     * @throws Exception\ResponseException
+     * @throws Exception\UnauthorizedException
+     */
+    public function getShippingLabelHeaders(string $shippingLabelId): ?array
+    {
+        $url = "retailer/shipping-labels/{$shippingLabelId}";
+        $options = [
+            'produces' => 'application/vnd.retailer.v10+pdf',
+        ];
+        $responseTypes = [
+            '404' => 'null',
+        ];
+
+        return $this->request('HEAD', $url, $options, $responseTypes);
+    }
+
     /**
      * Retrieves all event notification subscriptions for a given retailer. Each subscription may have different types
      * of events and a destination, which could either be a URL (for WEBHOOK) or a topic name (for GCP_PUBSUB).