add internal/timeline internal/xds module as stub help internal system for tweets message flow

internal/timeline/README.md

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+### Timeline(Timeline Message Flow System
+广场时间线消息流系统模块, 目前还没实现，先占个位置～

internal/timeline/timeline.go

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+// Copyright 2023 ROC. All rights reserved.
+// Use of this source code is governed by a MIT style
+// license that can be found in the LICENSE file.
+
+package timeline

internal/xds/README.md

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+### xDS(Recommended Discovery System）
+推荐发现系统模块，目前还没实现，先占个位置～

internal/xds/xds.go

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+// Copyright 2023 ROC. All rights reserved.
+// Use of this source code is governed by a MIT style
+// license that can be found in the LICENSE file.
+
+package xds