Allow distinct CAS url for server-side requests vs client-side redirects

[donquixote]

I am using Nextcloud with docker-compose, with another container providing a CAS mock server.

The user_cas app (and underlying phpCAS) has to build CAS urls for two purposes:

• For client-side redirects.
• For server-side curl requests (to validate a login).

In a docker scenario, without additional tricks, these urls can be different.
It could be useful to support distinct settings for these urls.

Btw, I considered to open this issue in https://github.com/apereo/phpCAS.
But we are not even using the latest version of that, so I couldn't tell if this is not already working in latest phpCAS.