Enable splice for freedom outbound (downlink only)

- Add outbound name
- Add outbound conn in ctx
- Refactor splice: it can be turn on from all inbounds and outbounds
- Refactor splice: Add splice copy to vless inbound
- Fix http error test
- Add freedom splice toggle via env var
- Populate outbound obj in context
- Use CanSpliceCopy to mark a connection
- Turn off splice by default

common/buf/copy.go

@@ -6,6 +6,7 @@ import (
 
 	"github.com/xtls/xray-core/common/errors"
 	"github.com/xtls/xray-core/common/signal"
+	"github.com/xtls/xray-core/features/stats"
 )
 
 type dataHandler func(MultiBuffer)
@@ -40,6 +41,17 @@ func CountSize(sc *SizeCounter) CopyOption {
 	}
 }
 
+// AddToStatCounter a CopyOption add to stat counter
+func AddToStatCounter(sc stats.Counter) CopyOption {
+	return func(handler *copyHandler) {
+		handler.onData = append(handler.onData, func(b MultiBuffer) {
+			if sc != nil {
+				sc.Add(int64(b.Len()))
+			}
+		})
+	}
+}
+
 type readError struct {
 	error
 }