Storage: Support generating public links to files

Currently libcloud has no way to generate (temporary) public download URLs for storage files, i.e. a rough equivalent to in generate_presigned_url() in boto3.

It would be nice if this functionality could be added, ideally with a less awful API that has some simple kwargs to specify the filename, disposition (inline/attachment) and content type instead of providing raw header values like it's the case in boto3.