Consider renaming "storage client" to "storage backend"

[vdusek]

The current naming around storage clients can be confusing - we have a top-level umbrella "storage client", individual clients for each storage type, and the Apify API client on top of that. This makes it harder to understand the actual role of these components.

It may be worth considering a more accurate and intuitive term, such as StorageBackend. These components represent pluggable & configurable backends for the storage layer, so it could make sense.

Additional context: https://apify.slack.com/archives/C06ENBV1B9Q/p1762439767261989.

[janbuchar]

Thanks for remembering this! Also, as discussed in the slack thread, we can rename the classes to *Backend and make *Client an alias that emits a deprecation warning if used.