Add support for OpenID Connect through Fedora

Hi! Fedora is going to remove support for OpenID in the infrastructure. Libravatar is one of the last external services using it. I have prepared a merge request to add support for authenticating with Fedora using OIDC. I can't send the MR here because this Gitlab instance does not allow me to create a repo in my namespace. You'll find the changes here:

https://github.com/abompard/ivatar/compare/master...oidc

It's one big commit plus two smaller ones that I had to do to make the unit tests pass.

How would you prefer to review this?

Thanks!