[5.x] Allow Cache Tags for POST requests

[marcorieser]

This PR allows Cache Tags to be enabled not only for GET but POST requests.

Backstory
I've detected that Cache Tags are not enabled for POST requests (Livewire subsequent requests), and had a little chat with Duncan about how I could support that in the Livewire addon. We came up with three approaches:

1. We add another condition to check if the request URL matches the Livewire update URL:

private function isEnabled()
{
    if (! config('statamic.system.cache_tags_enabled', true)) {
        return false;
    }

    if (request()->isLivePreview()) {
        return false;
    }

    // THIS
    if (request()->getRequestUri() === Livewire::getUpdateUri()) {
        return true;
    }

    // Only GET requests.
    return request()->method() === 'GET';
}

2. We make Tags/Cache::isEnabled() protected, I extend the cache tag in the Livewire addon, and overwrite that method with the check from the first approach
3. We allow all request types, since we have a dedicated check for Live Preview now (bab7822), which was probably the original reason for disabling it for POST requests. (Implemented approach)

Fixes: #12909