feat: Implement SOCKS4/a CONNECT

[HellLord77]

only CONNECT is supported (no BIND)
this could be added to mixed mode

[Copilot]

Pull request overview

Adds SOCKS4/SOCKS4a (CONNECT-only) support to the proxy stack by introducing new client/server handlers and wiring them into TCP handler factories and config enums.

Changes:

• Add Socks4TcpServerHandler and Socks4TcpClientHandler implementing SOCKS4/SOCKS4a CONNECT.
• Extend client/server config enums and TCP handler factories to support type: socks4.
• Add a basic SOCKS4 example configuration.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/tcp/tcp_server_handler_factory.rs	Wires ServerProxyConfig::Socks4 to the new SOCKS4 server handler.
src/tcp/tcp_client_handler_factory.rs	Wires ClientProxyConfig::Socks4 to the new SOCKS4 client handler (with optional local DNS resolution).
src/socks4_handler.rs	Implements SOCKS4/SOCKS4a parsing and request/response handling for CONNECT.
src/main.rs	Registers the new socks4_handler module for the binary crate.
src/lib.rs	Registers the new socks4_handler module for the library crate.
src/config/types/server.rs	Adds ServerProxyConfig::Socks4 { dns_enabled } and display formatting.
src/config/types/client.rs	Adds ClientProxyConfig::Socks4 { dns_enabled } and protocol name mapping.
examples/socks4_basic.yaml	Provides a sample YAML config using type: socks4.

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[cfal]

thanks for the PR! seems reasonable to support socks4/4a. some feedback:

• this should be part of socks_handler.rs - it should know how to handle 4 or 5
• dns_enabled is unnecessary, let's just always support it. we should simply return a NetLocation with a hostname and let the normal copy loop handle it

[HellLord77]

1. should it be transparent as mixed proxy? or a config to enable only 4/5/both servers? it should be noted that socks4 does not support any kind of auth, so a mixed handler might cause confusion (the USERID field could be implemented as a token auth, but not sure if that is the correct use)
2. dns_enabled should at least be necessary in client, in case user wants to tunnel through remote sokcs4 only server