RustlsConfig to be reloadable

[tisonkun]

Currently, the RustlsConfig can be configured with a fallback RustlsCertificate whose key and cert are preloaded bytes.

In reality, key and cert files can be updated periodically. Requiring a restart to reload the tls configs can be too expensive.

In GreptimeDB, there is a tech to reload files and the bytes will be read every time a new connection is established:

https://github.com/GreptimeTeam/greptimedb/blob/a283e13da744b6bbe3b08e7b0940ac2d3e03e4c6/src/servers/src/tls.rs#L169-L175

[tisonkun]

cc @sunli829 WDYT?

[tisonkun]

Currently make a reloadable solution by:

        Some(tls) => TcpListener::bind(listen_addr)
            .rustls(stream! {
                loop {
                    if let Ok(config) = load_tls_config(&tls) {
                        yield config;
                    }
                    tokio::time::sleep(CONFIG_RELOAD_INTERVAL).await;
                }
            })
            .into_acceptor()
            .await
            .map(AcceptorExt::boxed)
            .change_context_lazy(make_error),

As per example https://github.com/poem-web/poem/blob/master/examples/poem/tls-reload/src/main.rs.

Using https://docs.rs/notify/latest/notify/ to create a stream based on file changes should be possible.