fit: SSH connection interrupted

[luffywang1212-art]

This adds periodic WebSocket ping keepalive to RawSocketHandler by launching an asyncio task in proxy_open that sends a ping every 30 seconds. The ping triggers automatic pong responses from browsers, which counts as activity and prevents intermediate proxies (e.g., Nginx) from closing idle connections after ~60 seconds.
Fixes #557

[ryanlovett]

Hi @luffywang1212-art , thanks for your contribution. I think if we were to add this capability, it would at least need to be configurable, perhaps via server process option, and the default would be disabled.

However as far as I can tell, reverse proxies like nginx, haproxy, and envoy typically don't inject such websocket pings, and the keepalives are handled in the client or server. Curious what others might have to say...