Implements domain allowance

[yanc0]

Hi 👋

Thanks a lot for this radicale plugin. I plan to use it for my own needs. I use it with a SaaS mail provider hosting thousands of emails and domains. Of course, I don't want anyone hosted on this provider to authenticated against my radicale instance.

This is why I implemented another configuration to allow specific email domains only. This was inspired by the Gitea SMTP auth.

auth:
  allowed_domains = mydomain.tld, your.domain.tld

I took the liberty of adding some unit tests and to do some minor modifications (formatting, gitignore and adding some additionnal debugs)

Hope this pull request can help the community !
(please don't hesitate to critic my code, I'm not a python developer)

Have a great day !