chore: rename ambigous field proxy_port -> listen_port

[STRRL]

Signed-off-by: STRRL im@strrl.dev

here are other fields with similar name("proxy_ports") in

• https://github.com/STRRL/chaos-tproxy/blob/7845fee9c14c9395d3e87205c44874d6fddf379d/chaos-tproxy-proxy/src/raw_config.rs#L28
• https://github.com/STRRL/chaos-tproxy/blob/7845fee9c14c9395d3e87205c44874d6fddf379d/chaos-tproxy-controller/src/raw_config.rs#L7

And chaos_tproxy_proxy::proxy::http::server::HttpServer does not consider about them at all.

Rename it as listen_port would reduce the misunderstanding