network: cleanup NAT code and start on IPv6 support

shared/src/lib.rs

@@ -6,10 +6,17 @@ pub struct LaunchNetworkIpv4 {
     pub gateway: String,
 }
 
+#[derive(Serialize, Deserialize, Debug)]
+pub struct LaunchNetworkIpv6 {
+    pub address: String,
+    pub gateway: String,
+}
+
 #[derive(Serialize, Deserialize, Debug)]
 pub struct LaunchNetwork {
     pub link: String,
     pub ipv4: LaunchNetworkIpv4,
+    pub ipv6: Option<LaunchNetworkIpv6>,
 }
 
 #[derive(Serialize, Deserialize, Debug)]