Versioning or unique object identifiers in storage

## Feature Request

I want to synchronize my local files with different cloud storages. So, for this I would like to have some mechanism to check if a file in the cloud differs from my file. Unfortunately, I could not find any appropriate feature in libcloud.

I propose using the same hash method for all storage drivers and make it public, so anyone can check if file is the same in a local system and in any cloud.

However, this approach doesn't allow to determine where the file was changed, so, alternatively, we can use modify_time from `os.stat(file).st_mtime` or, maybe, you have a better idea.

Start by reviewing libcloud's storage-driver interfaces and how remote file metadata is exposed, then compare the proposed common hash with os.stat(file).st_mtime. The issue names no files or tests; done would require an agreed cross-driver identifier or change-detection contract and a clear public API.

Written by the indexing model from the issue text.