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.

Comienza revisando los puntos de entrada existentes de autenticación y plugins del repositorio, y compáralos después con los requisitos de los proveedores IndieAuth y Yarn.social descritos en el issue. Se considerará terminado cuando se haya acordado el alcance de la implementación y exista compatibilidad funcional con el inicio de sesión mediante IndieAuth para usuarios de Yarn.social sin requerir correo electrónico ni información personal.

Escrito por el modelo de indexación a partir del texto del issue.