Well, before this change the signature_certificate_file_list worked
only with filesystem paths.
 - i.e ['/home/user/public.pem']

Now it is possible to define urls as well.
 - i.e ['http://example.com/public.pem']

The url content will be downloaded to a temporary file before the
verification and automatically removed after.

The filesystem paths has higher priority than url, it is checked first.
If it does not find any valid certificate, then it will try
to download the certificates from the url provided.