cup bufferpool, add extendibility

**Is your feature request related to a problem? Please describe.**
简洁描述下新feature的内容

cup bufferpool cannot be extended. Add extendibility to it.

**Describe the solution you'd like**
描述下你想要什么样的解决方案

**Describe alternatives you've considered**
是否有其他替代方案

**Additional context**
可以辅助其他人理解你的信息

No file, test, or entry point is named. Start by locating the CUP bufferpool implementation and reading its current capacity and lifecycle behavior, then inspect related tests or usage examples. Done means the bufferpool can be extended and the behavior is covered by tests, but the required extension API and semantics need clarification first.

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