Write cmd-nse-istio-proxy

[edwarnicke]

Write a simple cmd-nse-istio-proxy which is basically a variation of cmd-icmp-responder but returns a kernel.Mechanism with:

1. SetRouteLocalNet(true) (see Add ROUTE_LOCALNET parameter to kernel.Mechanism api#134 )
2. SetIPTables4NatTemplate(tmpl) ( see Enable kernel.Mechanism to specify IP Tables ipv4 Nat Chains/Rules api#133 )

where:

tmpl := `-N NSM_PREROUTE
-A NSM_PREROUTE -j ISTIO_REDIRECT
-I PREROUTING 1 -p tcp -i {{ .NsmInterfaceName }} -j NSM_PREROUTE
-N NSM_OUTPUT
-A NSM_OUTPUT -j DNAT --to-destination {{ .NsmSrcIps[0] }}
-A OUTPUT -p tcp -s 127.0.0.6 -j NSM_OUTPUT
-N NSM_POSTROUTING
-A NSM_POSTROUTING -j SNAT --to-source ${NsmDstIPs[0]}
-A POSTROUTING -p tcp -o ${NSM_INTERFACE} -j NSM_POSTROUTING`

This depends on networkservicemesh/sdk-vpp#573

[edwarnicke]

#2 may aid in comprehension