[5.x] Static cache response statuses

[jasonvarga]

This PR allows developers to figure out if a given response was statically cached or not.

For example, in a custom middleware...

public function handle($request, $next)
{
    $response = $next($request);

    if ($response->wasStaticallyCached()) {
      // the request was previously cached and has just been retrieved.
    } else {
      // the request was not previously cached and has just been rendered from scratch
    }

    return $response;
}

The ResponseStatusTracker class exists purely because we can't put arbitrary properties on classes in PHP anymore. If we could, we'd just pop a $staticCacheResponseCode property on the Request via a macro. Alas.

The ResponseStatus enum will let us have different statuses for static cache types, similar to how Cloudflare does it. e.g. hit for when serving from cache, miss when served fresh, etc. This PR only introduces hit for now.

We considered simply adding X-Statamic-Static-Cache: hit headers to the response and let developers read those - but we don't want to expose that people are using Statamic. Some people don't like that.