Change PDO to singleton instance

- Add SQLite throwaway DB implementation
- Add integration tests for Custom class

tests/integration/TestDocument.php

@@ -0,0 +1,9 @@
+<?php declare(strict_types=1);
+
+namespace Test\Integration;
+
+class TestDocument
+{
+    public function __construct(public string $id = '', public string $value = '', public int $num_value = 0,
+                                public ?SubDocument $sub = null) { }
+}