Add a way to debug big_pipe cache tags

Problem/Motivation

When enabling debug_cacheability_headers, the X-Drupal-Cache-Tags header contains the cache tags used in the page, but it does not contain the cache tags of elements rendered by big_pipe.
This can be confusing because a developer can think some cache tag is missing from the page but it is not (as it is not always easy to remember which parts of the page are rendered by big_pipe).

Our specific use case is that we use big_pipe_sessionless in production and we want to make sure the non-big_pipe response cached by Varnish will have the appropriate cache tags without having to disable big_pipe on our development environment to check this.

Steps to reproduce

1. Enable big_pipe.
2. Enable debug_cacheability_headers.
3. X-Drupal-Cache-Tags does not contain cache tags specific to elements rendered by big_pipe.

Proposed resolution

big_pipe happens too late to add an HTTP header, but a solution could be to add an HTML comment at the end of the page containing this information.