Add a warning when multiple third-party modules implement "PurgeTagsHeader"

Problem/Motivation

If you use multiple third-party modules to purge remote cache services (ex: Varnish and a CDN), their respective "Tags Header sub-modules" will also be enable, hence each adding their headers with all the cache tags to Drupal responses.

Steps to reproduce

Enable and configure "purge_purger_http" and "varnish_purge", analyse a request, you see two huge identical content in headers : cache-tags & purge-cache-tags.

Proposed resolution

A warning should be displayed on the purge config page and in Drupal report status.
It could at least state that the response headers will be large and could lead to 500 errors on the web server or the reverse proxies.
It could also advise to disable all but one and set all reverse proxy to use the same header for cache tags.