[8.x] Call on_stats handler in Http stub callbacks

[funkjedi]

This updates the Http::fake() method to call the on_stats handler if possible when executing the stub callbacks. This change ensures that transferStats property gets populated on the faked Http response.

The transferStats property is a critical/vital component to some production code as it provides access to the underlying request that precipitated the response. For example if you have the following mixin.

use Psr\Http\Message\RequestInterface;

/** @mixin \Illuminate\Http\Client\Response */
class ResponseMixin
{
    public function getRequest()
    {
        return function (): ?RequestInterface {
            return optional($this->transferStats)->getRequest();
        };
    }
}

If the transferStats property is always null any code which is dependant on getting the request in this fashion becomes untestable without additional stubbing to call the on_stats handler.