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

app/proxyman/inbound/worker.go

@@ -60,6 +60,7 @@ func (w *tcpWorker) callback(conn stat.Connection) {
 	sid := session.NewID()
 	ctx = session.ContextWithID(ctx, sid)
 
+	var outbound = &session.Outbound{}
 	if w.recvOrigDest {
 		var dest net.Destination
 		switch getTProxyType(w.stream) {
@@ -74,11 +75,10 @@ func (w *tcpWorker) callback(conn stat.Connection) {
 			dest = net.DestinationFromAddr(conn.LocalAddr())
 		}
 		if dest.IsValid() {
-			ctx = session.ContextWithOutbound(ctx, &session.Outbound{
-				Target: dest,
-			})
+			outbound.Target = dest
 		}
 	}
+	ctx = session.ContextWithOutbound(ctx, outbound)
 
 	if w.uplinkCounter != nil || w.downlinkCounter != nil {
 		conn = &stat.CounterConnection{