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

testing/scenarios/vmess_test.go

@@ -1174,10 +1174,10 @@ func TestVMessGCMMuxUDP(t *testing.T) {
 	servers, err := InitializeServerConfigs(serverConfig, clientConfig)
 	common.Must(err)
 
-	for range "abcd" {
+	for range "ab" {
 		var errg errgroup.Group
 		for i := 0; i < 16; i++ {
-			errg.Go(testTCPConn(clientPort, 10240, time.Second*20))
+			errg.Go(testTCPConn(clientPort, 1024, time.Second*10))
 			errg.Go(testUDPConn(clientUDPPort, 1024, time.Second*10))
 		}
 		if err := errg.Wait(); err != nil {