Add L3 routing support

adapter/outbound.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"net"
 
+	"github.com/sagernet/sing-tun"
 	N "github.com/sagernet/sing/common/network"
 )
 
@@ -17,3 +18,8 @@ type Outbound interface {
 	NewConnection(ctx context.Context, conn net.Conn, metadata InboundContext) error
 	NewPacketConnection(ctx context.Context, conn N.PacketConn, metadata InboundContext) error
 }
+
+type IPOutbound interface {
+	Outbound
+	NewIPConnection(ctx context.Context, conn tun.RouteContext, metadata InboundContext) (tun.DirectDestination, error)
+}