Add configurable TCP keepalives to TCP listener and dialer

Exposes TCP keepalive configuration via listener and dialer metadata:

  keepalive: true
  keepalive.idle: 30s
  keepalive.interval: 10s
  keepalive.count: 6

When enabled, the OS sends keepalive probes after the idle period and
tears down the socket if the remote does not respond. This causes any
blocked Read() on a dead connection to return promptly, releasing
goroutines and memory — covering both legs of a CONNECT tunnel (the
inbound client connection via the listener, and the outbound upstream
connection via the dialer).

This is the correct alternative to a hard application-level read
deadline (SetReadDeadline) for detecting dead connections, as it
distinguishes truly dead connections from legitimately idle ones.

Relates to: https://github.com/go-gost/x/issues/91

dialer/tcp/dialer.go

@@ -47,6 +47,18 @@ func (d *tcpDialer) Dial(ctx context.Context, addr string, opts ...dialer.DialOp
 	conn, err := options.Dialer.Dial(ctx, "tcp", addr)
 	if err != nil {
 		d.logger.Error(err)
+		return conn, err
+	}
+
+	if d.md.keepalive {
+		if tc, ok := conn.(*net.TCPConn); ok {
+			tc.SetKeepAliveConfig(net.KeepAliveConfig{
+				Enable:   true,
+				Idle:     d.md.keepaliveIdle,
+				Interval: d.md.keepaliveInterval,
+				Count:    d.md.keepaliveCount,
+			})
+		}
 	}
 
 	conn = proxyproto.WrapClientConn(