feat: Expose the Notified future

[Marwes]

Motivation

I would like to avoid boxing the Notified future when storing it a struct in fede1024/rust-rdkafka#320 .

Solution

Exporting the type allows it to be named

[carllerche]

If we do this, we should probably create a future module in sync to avoid populating the sync docs w/ future types. This would match the error module pattern.

[Darksonn]

Closing as duplicate of #3840.