虚拟表

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当前MiniOB没有查询数据库内部状态的能力，比如当前正在执行的会话以及他们的状态、系统的配置信息、内部组件的统计信息等。

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实现虚拟表功能。
虚拟表的数据是在内存中的，给用户提供统一的抽象访问方式虚拟表，用户可以访问内存中的数据。
比如系统配置，提供虚拟表实现查询方式： select * from miniob.__config;
在 miniob.__config的存储引擎中，返回配置数据访问的接口。
此功能依赖 #345 提供一个抽象的引擎访问接口。

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OceanBase 的实现参考 https://github.com/oceanbase/oceanbase/tree/develop/src/observer/virtual_table

还可以参考 MySQL information_schema 的实现。

Start by reading the abstract engine access interface proposed in issue #345, then compare the requested behavior with OceanBase's virtual_table implementation and MySQL information_schema. Define the scope of the first virtual table, such as miniob.__config, and verify that its in-memory data can be queried through the example SQL statement.