interface: Add download_url method and implement it in some backends
Some objstorage backends, typically cloud based ones like azure or s3, can generate a temporary direct download link for a stored object.
So add a download_url
method to objstorage interface and implement it
in azure, http, libcloud and seaweedfs backends.
Related to swh-vault#885 (closed).
Depends on !155 (merged)
Edited by Antoine Lambert