chore: add early conn interface to decrease unneeded write

common/net/sing.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"net"
 
+	"github.com/sagernet/sing/common"
 	"github.com/sagernet/sing/common/bufio"
 	"github.com/sagernet/sing/common/network"
 )
@@ -16,6 +17,13 @@ type ExtendedConn = network.ExtendedConn
 type ExtendedWriter = network.ExtendedWriter
 type ExtendedReader = network.ExtendedReader
 
+func NeedHandshake(conn any) bool {
+	if earlyConn, isEarlyConn := common.Cast[network.EarlyConn](conn); isEarlyConn && earlyConn.NeedHandshake() {
+		return true
+	}
+	return false
+}
+
 // Relay copies between left and right bidirectionally.
 func Relay(leftConn, rightConn net.Conn) {
 	_ = bufio.CopyConn(context.TODO(), leftConn, rightConn)