Describe the release item

When a session is closed, subscriber callbacks are dropped, causing their channel handler to be closed. However, AdvancedSubscriber callback is not only owned by the session ; it is cloned to be put in the state.
https://github.com/ZettaScaleLabs/zenoh/blob/11c29786c263a230a3fca2bd60e960be758f0026/zenoh-ext/src/advanced_subscriber.rs#L686-L686

A solution to this issue could be to allow registering "closing callbacks" into Session, and use one callback to drop the subscriber.