feat:已实现drop tabl全功能

[qizidehenji]

本次改动新增 drop table 语法支持：

1. 新增 drop table 语句语法解析
2. 实现 DropTableStmt 语句类
3. 新增 DropTableExecutor 执行器
4. 在 CommandExecutor 中注册 drop table 执行分支
5. 实现 Db 层 drop_table 底层逻辑，可删除内存表+磁盘元数据、数据文件

测试结果：

• create table / insert / select 功能正常
• drop table 可以正常删除表
• 重启服务后，删除效果永久生效

[CLAassistant]

All committers have signed the CLA.

[qizidehenji]

Hello OceanBase MiniOB maintainers,

This PR (#639) fully implements the DROP TABLE statement capability.

✅ All 18 CI build and test checks passed
✅ CLA signing completed
✅ Code clang-format style fully compliant

It is now ready for your code review. Looking forward to your approval and merge.

Thank you for your time!