implemented snapshot diff; started work on admin settings

lib/Entity/Snapshot.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace OCA\GroupfolderFilesystemSnapshots\Entity;
+
+use JsonSerializable;
+
+class Snapshot implements JsonSerializable {
+    /** @var string */
+    private $id;
+
+    public function __construct(string $id) {
+        $this->id = $id;
+    }
+
+    public function jsonSerialize(): mixed {
+		return [
+			'id' => $this->id
+		];
+	}
+}