Add support for Simple Proxy Protocol (UDP)

[mschneider82]

a lightweight protocol developed specifically for UDP

https://developers.cloudflare.com/spectrum/how-to/enable-proxy-protocol/#enable-simple-proxy-protocol-for-udp
https://developers.cloudflare.com/spectrum/reference/simple-proxy-protocol-header/

[delthas]

FWIW Cloudflare also supports the Proxy Protocol v2 for UDP. https://developers.cloudflare.com/spectrum/how-to/enable-proxy-protocol/#enable-proxy-protocol-v2-for-tcpudp