backport interface

Closes #3500683

core/lib/Drupal/Core/Cache/CacheOptionalInterface.php

+<?php
+
+namespace Drupal\Core\Cache;
+
+/**
+ * Indicates that caching is optional.
+ *
+ * This interface is backported to Drupal 10.5 to make it easier to support it,
+ * There are no usages in Drupal core before 11.2
+ */
+interface CacheOptionalInterface {
+
+}