New plugin use-case/type of plugin: Credentials Guard

Hi 👋,

First of all, thanks for providing _Answer_ and congrats for joining Apache 👍

I'm having a plugin use-case in mind I'd like to work on, that is - as far as I'm able to see - [currently not supported](https://answer.apache.org/docs/plugins/#plugin-type).

**Idea/Use-Case**

I'd like to implement a plugin that checks user submitted content for security sensitive information like credentials and asks
the user if the content really should be submitted as is.
Submitting such kind of information if fairly easy especially if you're copy & pasting code snippets (totally unrelated to Answer itself obviously).

This would allow to not even persist sensitive data in case it's not intended by the user. Persisiting sensitive information is an issue as Answer keeps the content history - which is really great in general (I really like the diff functionality ❤️) but not in case you submit something that better shouldn't have been.
To detect credentials potentially, I was thinking about using something like e.g. [DeepPass](https://github.com/GhostPack/DeepPass).

_Additional thoughts_
* it potentially involves both frontend and backend functionality
* needs to intervene (optionally with user consent) before data gets persisted
* should involve scanning user questions, answers and comments, basically all user submitted content

Looking forward for feedback 😃

issue にリンクされている plugin-type のドキュメントから始め、repository に存在する plugin types と extension points を調査します。永続化前に質問、回答、コメントをスキャンする範囲を、ユーザーの任意の同意とコンテンツ履歴を含めて定義します。完了条件は、合意された plugin type と実装範囲があることです。

索引モデルが issue の本文から書いたものです。