update download function

[fenix-hub]

• files from storage are now retrieved by default as a PoolByteArray
• it is now possible to define wether a file is supposed to be saved locally or not:
• • bucket.download("images/image.png", "user://images/image.png") retrieve the file from storage as a PoolByteArray and save to the defined path
• • bucket.download("images/image.png") will just retrieve the file as Bytes. To get the bytes, the StorageTask.data property will hold the content as bytes.