Proxy support for Socket Mode (WS)

[mehr74]

I am trying to use the proxy connector example to use socket api connection. The wss socket does not use the proxy and does not proceed.
I believe the following line should be changed, and client_async_tls should be used instead when the client uses a proxy connector.

slack-morphism-rust/src/hyper_tokio/socket_mode/tungstenite_wss_client.rs

Line 106 in d392361

match connect_async(&url_to_connect).await {

[abdolence]

Hey.

Existing proxy support was intended only for Slack client, not for Socket Mode/Web Sockets.

Reading this
snapview/tungstenite-rs#177 I'm not sure if this works for HTTP/HTTPS proxies.

Would be great if anyone who needs proxies could test it and prepare PR, since I don't have that kind of needs myself.