feat: Add data plane support to FFI (#2287)

1. Overview

This PR adds data plane APIs to easytier-ffi:

TCP Outbound:

- data_plane_tcp_connect
- data_plane_tcp_read
- data_plane_tcp_write
- data_plane_tcp_close

TCP Listener:

- data_plane_tcp_bind
- data_plane_tcp_accept
- data_plane_tcp_listener_close

UDP:

- data_plane_udp_bind
- data_plane_udp_send_to
- data_plane_udp_recv_from
- data_plane_udp_close

2. Key Changes

The main changes are focused on:

- easytier-contrib/easytier-ffi/src/lib.rs: Added FFI interfaces;
  made ERROR_MSG thread-safe.

- easytier/src/gateway/socks5.rs: Bridges the data plane to the
  existing Socks5 server logic.
  - Added EasyTierUdpSocket, mainly wrapping ref-counting and
    critical object (e.g., Socks5EntrySet) hold & drop logic,
    and exposing common fields (e.g., local_addr).
  - Extended Socks5Server functionality to expose TCP and UDP
    socket creation interfaces for FFI calls.

- Other files: Mostly pass-through logic.

- Added a relatively large Go usage example.

easytier/src/instance_manager.rs

@@ -1,3 +1,5 @@
+#[cfg(feature = "ffi-dataplane")]
+use crate::launcher::{DataPlaneTcpListener, DataPlaneTcpStream, DataPlaneUdpSocket};
 use dashmap::DashMap;
 use std::fmt::{Display, Formatter};
 use std::{collections::BTreeMap, path::PathBuf, sync::Arc};
@@ -171,6 +173,59 @@ impl NetworkInstanceManager {
         tokio::runtime::Runtime::new()?.block_on(self.collect_network_infos())
     }
 
+    #[cfg(feature = "ffi-dataplane")]
+    pub async fn data_plane_tcp_connect(
+        &self,
+        instance_id: &uuid::Uuid,
+        dst_addr: std::net::SocketAddr,
+        timeout: std::time::Duration,
+    ) -> Result<DataPlaneTcpStream, anyhow::Error> {
+        let instance = self
+            .instance_map
+            .get(instance_id)
+            .ok_or_else(|| anyhow::anyhow!("instance {} not found", instance_id))?;
+        instance.data_plane_tcp_connect(dst_addr, timeout).await
+    }
+
+    #[cfg(feature = "ffi-dataplane")]
+    pub async fn data_plane_tcp_bind(
+        &self,
+        instance_id: &uuid::Uuid,
+        local_port: u16,
+        timeout: std::time::Duration,
+    ) -> Result<DataPlaneTcpListener, anyhow::Error> {
+        let instance = self
+            .instance_map
+            .get(instance_id)
+            .ok_or_else(|| anyhow::anyhow!("instance {} not found", instance_id))?;
+        instance.data_plane_tcp_bind(local_port, timeout).await
+    }
+
+    #[cfg(feature = "ffi-dataplane")]
+    pub async fn data_plane_udp_bind(
+        &self,
+        instance_id: &uuid::Uuid,
+        local_port: u16,
+        timeout: std::time::Duration,
+    ) -> Result<DataPlaneUdpSocket, anyhow::Error> {
+        let instance = self
+            .instance_map
+            .get(instance_id)
+            .ok_or_else(|| anyhow::anyhow!("instance {} not found", instance_id))?;
+        instance.data_plane_udp_bind(local_port, timeout).await
+    }
+
+    #[cfg(feature = "ffi-dataplane")]
+    pub fn data_plane_wait_runtime_handle(
+        &self,
+        instance_id: &uuid::Uuid,
+        timeout: std::time::Duration,
+    ) -> Option<tokio::runtime::Handle> {
+        self.instance_map
+            .get(instance_id)
+            .and_then(|inst| inst.wait_runtime_handle(timeout))
+    }
+
     pub async fn get_network_info(
         &self,
         instance_id: &uuid::Uuid,