Resolve #3414287 "Big pipe"

Closes #3414287

core/lib/Drupal/Core/Session/ResponseKeepSessionOpenInterface.php

+<?php
+
+namespace Drupal\Core\Session;
+
+/**
+ * Indicates that sessions for this response should be kept open after sending.
+ *
+ * By default, Drupal closes sessions as soon as the response is sent. If
+ * a response implements this interface, Drupal will skip this behavior and
+ * assume that the session will be closed manually later in the request.
+ *
+ * @see Drupal\Core\StackMiddleware\Session
+ * @see Drupal\big_pipe\src\Render\BigPipeResponse
+ * @internal
+ */
+interface ResponseKeepSessionOpenInterface {}