IndieAuth Support

I would like to one day set up an Answers instance for [Yarn.social](https://yarn.social) for the (small, but) growing community.

In order to continue to project the privacy of users it would be nice to have support for [IndieAuth](https://indieauth.com/), which works a bit like OAuth but is a bit simpler and designed to support "logging in via your own domain".

As all instances of Yarn.social pods (`yarnd`) today do not require any kind of email address or personal information and are also valid IndieAuth providers, this is ideal as users in the community can just login with their existing credentials against their domain/pod.

Thank you for considering this feature! 🙏 I fully support many of the aspects of the [IndieWeb](https://indieweb.org/) such as IndieAuth as they tend to err on the side of privacy and owning your own data.

まず、リポジトリに既存する認証およびプラグインのエントリポイントを確認し、issue に記載されている IndieAuth および Yarn.social のプロバイダー要件と比較します。実装範囲について合意され、メールアドレスや個人情報を必要とせずに Yarn.social ユーザーが IndieAuth でログインできるようになれば完了です。

索引モデルが issue の本文から書いたものです。