Add fake port option

[Protryon]

Adds a fake port option for wonky firewall shenanigans.

[ylow]

Apologies as you might have heard we just got acquired by HuggingFace. Things have been hectic for a while, but now I have bandwidth to review this. Thanks for the patience!

[rrauch]

How about adding a new, optional advertised_port field to NFSTcpListener and then change how local_port in RPCContext is assigned?

Something like this:

pub struct NFSTcpListener<T: NFSFileSystem + Send + Sync + 'static> {
    listener: TcpListener,
    port: u16,
    advertised_port: Option<u16>,
    arcfs: Arc<T>,
    mount_signal: Option<mpsc::Sender<bool>>,
}

...

async fn handle_forever(&self) -> io::Result<()> {
        loop {
            let (socket, _) = self.listener.accept().await?;
            let context = RPCContext {
                local_port: self.advertised_port.unwrap_or(self.port),
                client_addr: socket.peer_addr().unwrap().to_string(),
...                

I think the ability to advertise a different port from the actual listening port is very useful, especially when dealing with Windows clients.

[ylow]

Can we maybe split this into 2 PRs? This looks like it has 2 pieces. One which implements NLM4 protocol (thanks!), and another for a port number.