Feature request: Single Sign On (SSO) support

# Use case

When selfhosting a number of services, having many different credentials does not make much sense and becomes cumbersome, so I use an SSO provider to login instead.

# Your idea for a solution

Support SSO providers via [Open ID Connect](https://openid.net/connect/) to login (and maybe signup?) to maddy.

- [ ] I'm willing to help with the implementation

Start by reading maddy's existing authentication and account-creation paths, then review the linked OpenID Connect specification and determine the required provider flow. The issue leaves signup support open, so confirm whether the scope covers login only or both login and signup before defining completion.

Written by the indexing model from the issue text.