Added endpoint to truncate contact list (#21)

src/Request/Contactlists/ContactlistEndpoint.php

@@ -25,4 +25,9 @@ public function get(int $listId): Contactlist
         return new Contactlist($this->api, $listId);
     }
 
+    public function truncate(int $listId): ContactlistTruncate
+    {
+        return new ContactlistTruncate($this->api, $listId);
+    }
+
 }

src/Request/Contactlists/ContactlistTruncate.php

@@ -0,0 +1,50 @@
+<?php declare(strict_types=1);
+
+namespace SmartEmailing\v3\Request\Contactlists;
+
+use SmartEmailing\v3\Api;
+use SmartEmailing\v3\Exceptions\InvalidFormatException;
+use SmartEmailing\v3\Request\AbstractRequest;
+
+class ContactlistTruncate extends AbstractRequest
+{
+    /** @var int */
+    private $listId;
+
+    public function __construct(Api $api, int $listId)
+    {
+        parent::__construct($api);
+
+        $this->listId = $listId;
+    }
+
+    protected function method(): string
+    {
+        return 'POST';
+    }
+
+    protected function endpoint(): string
+    {
+        return 'contactlists/' . $this->listId . '/truncate';
+    }
+
+    protected function options(): array
+    {
+        return [
+            'json' => $this->jsonSerialize()
+        ];
+    }
+
+    private function toArray(): array
+    {
+        return [];
+    }
+
+    /**
+     * @return array
+     */
+    public function jsonSerialize()
+    {
+        return $this->toArray();
+    }
+}