OpenID Connect authentication backend

[azmeuk]

I suggest implementing an OIDC authentication backend that would deal with bearer tokens.

Should this belong in the main repository, or in a dedicated plugin elsewhere?
What do you think?

[MatthewHana]

The practice with Radicale appears to be that auth mechanisms are separate plugins (have a look at Arvedui/radicale-dovecot-auth for example).

Also there is a long backlog of pull requests, so it might be faster to implement it as a plugin.

[sphakka]

Interested in this, available for help/test. Maybe as a starting point, Modoboa provides a simple token-based mechanism.