add support for Google Cloud Storage

[shcode]

Add support for GCS

[antongorodezkiy]

Thank you for sharing your work @shcode, will try to use it

[antongorodezkiy]

Had to make few modifications for my case and the special one to avoid the exception

Serialization of 'Closure' is not allowed

I had to explicitly set serializable properties in GCS.php to exclude $filesystem from the serialization:

// NOTE: need to explicitly set serializable properties
        public function __sleep()
        {
            return ['attachedFile'];
        }

        public function __wakeup()
        {
            $this->filesystem = StaplerExtended::getGCSClientInstance($this->attachedFile);
        }