Super Cache: Add a form for setting up caching of static files by proxies and clients

The plugin should have a form that allows the user to configure caching for their static files. We already have a CDN page, so this is a natural extension of that.

Adding the directives can be easy enough, at least with some web servers. Apache servers only need a modified .htaccess file, which the plugin can do already.

Resources:
* https://fornex.com/en/help/htaccess-cache/
* https://www.ecenica.com/support/answer/htaccess-cache-static-content/
* https://imagekit.io/blog/ultimate-guide-to-http-caching-for-static-assets/