Database DDL & Migration Support for plugins

## Is your feature request related to a problem? Please describe

Apache Answer should support a way for plugin devs to have DDL and migration specific to their plugins / code.

There are plenty of use cases for this.
Most basic use cases is their tables would act as meta for their plugins.

## Describe the solution you'd like

Apache Answer should support a way for plugin devs to have DDL and migration specific to their plugins / code.

## Describe alternatives you've considered

Alternative is plugin having its own database. This idea sucks when you have use cases of joins between answer db to plugin db.

未指定文件、测试或入口点。首先定位插件生命周期、现有的数据库 DDL 以及迁移机制。当插件开发者能够定义并运行插件专属的 schema 变更，同时将这些表保留在 Apache Answer 数据库中以便进行 join 时，即表示完成。

由索引模型根据 Issue 内容生成。