feat: initial SOCKS5 proxy with chain support

Asyncio-based SOCKS5 server that tunnels connections through
configurable chains of SOCKS5, SOCKS4/4a, and HTTP CONNECT proxies.
Tor integration via standard SOCKS5 hop.

config/example.yaml

@@ -0,0 +1,15 @@
+# s5p configuration
+
+listen: 127.0.0.1:1080
+timeout: 10
+log_level: info
+
+# Proxy chain -- connections tunnel through each hop in order.
+# Supported protocols: socks5://, socks4://, http://
+#
+# Example: route through Tor, then an external SOCKS5 proxy
+chain:
+  - socks5://127.0.0.1:9050          # Tor
+  # - socks5://user:pass@proxy:1080  # authenticated SOCKS5
+  # - socks4://proxy:1080            # SOCKS4/4a
+  # - http://user:pass@proxy:8080    # HTTP CONNECT