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

proxy/dokodemo/dokodemo.go

@@ -102,11 +102,10 @@ func (d *DokodemoDoor) Process(ctx context.Context, network net.Network, conn st
 	}
 
 	inbound := session.InboundFromContext(ctx)
-	if inbound != nil {
-		inbound.Name = "dokodemo-door"
-		inbound.User = &protocol.MemoryUser{
-			Level: d.config.UserLevel,
-		}
+	inbound.Name = "dokodemo-door"
+	inbound.SetCanSpliceCopy(1)
+	inbound.User = &protocol.MemoryUser{
+		Level: d.config.UserLevel,
 	}
 
 	ctx = log.ContextWithAccessMessage(ctx, &log.AccessMessage{