Adds on_disconnect handler to rumqttd.

[nathansizemore]

We're using rumqttd against devices that uses v4 protocol and do not always disconnect gracefully. It appears in this circumstance, the broker never reports a disconnect notification as would be expected here:

link_tx.subscribe("#").unwrap();

loop {
    let notification = match link_rx.recv().unwrap() {
        Some(v) => v,
        None => continue,
    };

    match notification {
        Notification::Forward(forward) => println!("forward"),
        Notification::Disconnect(d, props) => println!("disconnect: \n{d:#?}\n{props:#?}"),
        v => println!("{v:?}"),
    }
}

We also start processes based on a client id, so not having access to that information when the client is no longer connected is a pain point this also solves by passing in the id for the disconnected client to the handler.

This is related to #775 .

Type of change

New feature (non-breaking change which adds functionality)

Checklist:

• [x] Formatted with cargo fmt
• [x] Make an entry to CHANGELOG.md if it's relevant to the users of the library. If it's not relevant mention why.

[nathansizemore]

closing in favor of another PR will impl a little differently.