Implement SSH Phase 13.1: Enterprise-level security configuration

- Add ssh_security_config.rs module (150 lines)
- Define SshSecurityConfig structure (GatewayPorts, PermitOpen, etc.)
- Implement enterprise_default() and development_default()
- Add validate_tcpip_forward_request() security validation
- Add validate_direct_tcpip_channel() security validation
- Integrate SshSecurityConfig into server.rs
- Add SSH_MSG_GLOBAL_REQUEST handling in service loop
- Initialize PortForwardManager for port forwarding
- Create data/ssh_config.json example file
- Support session counting (increment/decrement)
- All compilation tests passed successfully

data/ssh_config.json

@@ -0,0 +1,13 @@
+{
+  "ssh_server": {
+    "port": 2024,
+    "bind_address": "127.0.0.1",
+    "security": {
+      "gateway_ports": false,
+      "permit_open": ["localhost:*"],
+      "allow_tcp_forwarding": true,
+      "max_sessions": 10,
+      "connect_timeout": 30
+    }
+  }
+}