Implement queries into list variables

src/Appwrite/Utopia/Database/Validator/Queries/Variables.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace Appwrite\Utopia\Database\Validator\Queries;
+
+use Appwrite\Utopia\Database\Validator\Queries\Base;
+
+class Variables extends Base
+{
+    public const ALLOWED_ATTRIBUTES = [
+        'key'
+    ];
+
+    /**
+     * Expression constructor
+     *
+     */
+    public function __construct()
+    {
+        parent::__construct('variables', self::ALLOWED_ATTRIBUTES);
+    }
+}