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/outbound/handler.go

@@ -274,7 +274,12 @@ func (h *Handler) Dial(ctx context.Context, dest net.Destination) (stat.Connecti
 	}
 
 	conn, err := internet.Dial(ctx, dest, h.streamSettings)
-	return h.getStatCouterConnection(conn), err
+	conn = h.getStatCouterConnection(conn)
+	outbound := session.OutboundFromContext(ctx)
+	if outbound != nil {
+		outbound.Conn = conn
+	}
+	return conn, err
 }
 
 func (h *Handler) getStatCouterConnection(conn stat.Connection) stat.Connection {