Enable kernel.Mechanism to specify IP Tables ipv4 Nat Chains/Rules

Introduce new constant for parameters for kernel.Mechanism:

IPTABLES4_NAT_TEMPLATE = "iptables__nat_template"

And provide helpers:

# Getters and Setters
func (m *Mechanism) SetIPTables4NatTemplate(tmpl string) *Mechanism {}
func (m *Mechanism) GetIPTables4NatTemplate(tmpl string) string {}

# Helper to evaluate the iptables template using a *networkserviceConnection
func (m *Mechanism) EvaluateIPTables4NatTemplate(conn *networkservice.Connection) {}

The ip tables nat template is expected to be a golang text/template to inject a struct with fields:

• NsmInterfaceName - the name of the interface from conn.GetMechanism().GetInterfaceName()
• NsmSrcIPs - the ipv4 addresses from conn.GetContext().GetIPContext(). GetSrcIpAddrs()
• NsmDstIPs - the ipv4 addresses from conn.GetContext().GetIPContext().GetDstIPAddrs()

Example template:

-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

Start in pkg/api/networkservice/mechanisms/kernel and inspect the kernel.Mechanism API, then trace the networkservice.Connection accessors named in the issue. Define the constant and helpers around the specified NAT template fields, and verify that evaluation produces the requested iptables rules from the connection data.