[Feature Request] Generic OIDC/OAuth2 support

[hazzuk]

Is your feature request related to a problem? Please describe.
Calibre-web only supports SSO OAuth with external providers Google and Github.

Describe the solution you'd like
Adding a generic implementation would allow for authentication with any provider, most importantly allowing self-hosted solutions like Authentik, Authelia and Keycloak.

Additional context
This has been a topic of discussion for many years on Calibre-web, but the maintainer has said quite definitively that "We will not extend Calibre-Web with any more login abilities".

A Pull Request was even made with this functionality, but sadly was never merged.

[crocodilestick]

This is super interesting but something I'm not particularly versed in. I'm stretched pretty thin atm but if someone can make a PR for CWA using the implementation mentioned in the PR you linked I'll include it in the next release ASAP.