Add sFlow support like goflow2 already has

[Nachtfalkeaw]

Hello,

I am using Hardware Loadbalancer whch can only export sFlow and no other protocols.
As the goflow2 package supports sflow I assume the code part is already in flp.

For that reason I would like to see sFlow as a supported protocol. In may case it contains more information like status codes and other header and http information and probably for that reason the vendor used it and not netflow/ipfix.