Add a CAS connector

[lucassardois]

Add a CAS connector to Dex.

Is your feature request related to a problem?

Yes, we want to connect Dex to our LDAP (done), an OpenID Connect provider (done) and a CAS system.

Describe the solution you'd like to see

A new connector for CAS systems.

Describe alternatives you've considered

Write a middleware acting as an OpenID Connect provider between the CAS and Dex. Dex -- (OpenID Connect) -> Middleware -- (magic) -> CAS

Additional context

CAS systems are quite used by universities around the globe, as such it would be a cool feature I think.

[sagikazarmark]

I think it would be a good fit, but also that we might want to wait until external connector support is added.

[hellobiek]

really need it

[lxkaka]

CAS connector is necessary

[emanuelegissi]

A CAS connector is desirable. How can we help?