Without modifying the codebase, is there a way that I could leverage my own OpenID authentication server? I would think it would be a config setting. Similar to how we can config Google Client and Secret, we could do the same but also add the auth server address.
Yes, there are two ways:
Use a custom external authentication server (recommended): https://github.com/Squidex/squidex/blob/master/backend/src/Squidex/appsettings.json#L619
Replace the whole authentication system (not recommended and very complicated).
If only one authentication method is configured and this configuration is an external server the login page will redirect to your server automatically so the user experience is more or less the same.