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.

ファイル、テスト、エントリポイントは指定されていません。まず、plugin lifecycle、既存の database DDL、migration mechanism の場所を確認してください。Plugin 開発者が plugin 固有のスキーマ変更を定義して実行でき、これらのテーブルを join のために Apache Answer データベース内に保持できれば完了です。

索引モデルが issue の本文から書いたものです。