Add support for client-writable signals

[tqwewe]

Currently, server signals are read only on the client, and cannot be updated other than by the server.

It would be nice to have writable signals from the client.

I assume this would involve a similar API to the server_fn macro, with a mpsc sender and receiver:

#[server_signal(Counter)]
pub async fn counter(count: i32, tx: Sender<i32>, mut rx: Receiver<i32>) -> Result<(), ServerFnError> {
    while let Some(new_count) = rx.receive().await? {
        println!("new count is {new_count}");
        tx.send(new_count + 1).await?;
    }
    
    Ok(())
}

[Boscop]

@tqwewe Yes, please! 🙏
It would be very useful to have a separate kind of signal that notifies the server of changes on the client side (from UI controls).
Have you looked into this more? 🙂

[tqwewe]

@Boscop I haven't had a need for this in any of my projects yet, so haven't taken the time to look into it. If I do need it, I certainly will implement it, but sadly don't have the time to implement it currently. PRs are absolutely welcome for this if anyone wants to give it a shot.