Addons for external cache systems

[luisherranz]

Hey guys, I would love to start a discussion to see if it would be possible to add an addon system, as proposed by @richardkorthuis when I mentioned this.

External cache systems that could benefit from this:

• CDNs like Cloudflare, Fastly, KeyCDN, StackPath and so on.
• Server caches like Varnish, Nginx Cache and so on.
• Hosting providers, like WordPress VIP, Flywheel, WP Engine and so on.

I guess that what needs to happen is:

• The plugin doesn't need to save the response and headers in the database anymore, only the relations. I guess this can be optional, depending on each addon and even website it could make sense to turn this on or off.
• The plugin should allow other systems (addons) to use filters/actions when it is:
  • Saving resources. For example, for setting Cache Tags before the response is sent.
  • Purging the resources, to purge them as well in the external system.

I was also thinking if it would be possible to integrate this with page cache plugins like SuperCache because those are the "owners" of the advanced-cache.php file and therefore the only ones that can bypass the WordPress load.