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.

.gitignore

@@ -43,3 +43,6 @@ easytier-gui/src-tauri/*.sys
 
 .direnv
 .flake-profile
+
+# contrib
+go.sum