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.

首先检查 repository 中现有的身份验证和插件入口点，然后将其与 issue 中描述的 IndieAuth 和 Yarn.social provider 要求进行比较。当实现范围达成一致，并且 Yarn.social 用户无需提供电子邮件或个人信息即可使用 IndieAuth 登录时，即视为完成。

由索引模型根据 Issue 内容生成。