add subscription process endpoint

src/Entities/Subscription.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace D3\KlicktippPhpClient\Entities;
+
+use Doctrine\Common\Collections\ArrayCollection;
+
+class Subscription extends ArrayCollection
+{
+    public function getId(): string
+    {
+        return $this->get('id');
+    }
+}