Function proxy_protocol_v1_header 

pub fn proxy_protocol_v1_header(
    client_addr: &SocketAddr,
    server_addr: &SocketAddr,
) -> String

Build a PROXY protocol v1 header line.

Format: PROXY TCP4 <src_ip> <dst_ip> <src_port> <dst_port>\r\n or PROXY TCP6 for IPv6.