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/vmess/inbound/inbound.go

@@ -256,10 +256,8 @@ func (h *Handler) Process(ctx context.Context, network net.Network, connection s
 	}
 
 	inbound := session.InboundFromContext(ctx)
-	if inbound == nil {
-		panic("no inbound metadata")
-	}
 	inbound.Name = "vmess"
+	inbound.SetCanSpliceCopy(3)
 	inbound.User = request.User
 
 	sessionPolicy = h.policyManager.ForLevel(request.User.Level)