[HttpCache] Explain how to extend the HttpCache class

[HypeMC]

Followup to #16402.

In versions of Symfony prior to 5.4 the caching kernel was enabled by wrapping the default kernel into it inside the front controller, eg:

// public/index.php
use App\CacheKernel;
use App\Kernel;

// ...
$kernel = new Kernel($_SERVER['APP_ENV'], (bool) $_SERVER['APP_DEBUG']);
// Wrap the default Kernel with the CacheKernel one in 'prod' environment
if ('prod' === $kernel->getEnvironment()) {
    $kernel = new CacheKernel($kernel);
}

Starting from version 5.4 this was changed to be done through the configuration:

# config/packages/framework.yaml
when@prod:
    framework:
        http_cache: true

However, it was never explained how the caching kernel can be extended in this case. Decorating the http_cache service seems like the best option.

[OskarStark]

I would love to see a follow up PR against 6.2 adding how this can be done using an attribute 👍

[HypeMC]

Done, see #17975.

[OskarStark]

Thank you @HypeMC.