Add support for IPv6 within VPN (#1061)

* add flake.nix with nix based dev shell
* add support for IPv6
* update thunk

---------

Co-authored-by: sijie.sun <sijie.sun@smartx.com>

easytier/src/easytier-core.rs

@@ -148,6 +148,13 @@ struct NetworkOptions {
     )]
     ipv4: Option<String>,
 
+    #[arg(
+        long,
+        env = "ET_IPV6",
+        help = t!("core_clap.ipv6").to_string()
+    )]
+    ipv6: Option<String>,
+
     #[arg(
         short,
         long,
@@ -615,6 +622,12 @@ impl NetworkOptions {
             })?))
         }
 
+        if let Some(ipv6) = &self.ipv6 {
+            cfg.set_ipv6(Some(ipv6.parse().with_context(|| {
+                format!("failed to parse ipv6 address: {}", ipv6)
+            })?))
+        }
+
         if !self.peers.is_empty() {
             let mut peers = cfg.get_peers();
             peers.reserve(peers.len() + self.peers.len());