Implement filter_map on watcher

[rklaehn]

If you want to wait for a certain state, it would be neat to be able to not just map but also filter and filter_map on a watcher.

E.g. when waiting for a direct connection, this is currently only possible by manually looping through the stream:

let on_connected = |ep: Endpoint, conn: Connection| async move {
    let Ok(id) = conn.remote_node_id() else {
        return Err(io::Error::other("unable to get node id"));
    };
    let Some(watcher) = ep.conn_type(id) else {
        return Err(io::Error::other("unable to get conn_type watcher"));
    };
    let mut stream = watcher.stream();
    while let Some(status) = stream.next().await {
        if let ConnectionType::Direct { .. } = status {
            return Ok(());
        }
    }
    Err(io::Error::other("connection closed before becoming direct"))
};

Having filter_map would simplify this to:

let on_connected = |ep: Endpoint, conn: Connection| async move {
    let Ok(id) = conn.remote_node_id() else {
        return Err(io::Error::other("unable to get node id"));
    };
    let Some(watcher) = ep.conn_type(id) else {
        return Err(io::Error::other("unable to get conn_type watcher"));
    };
    watcher.filter(|t| t == ConnectionType::Direct).initialized().await;
};