CAS 3 support and automatic e-mail extraction

I'm not an expert php developer but I managed to add support for CAS 3 in glpi in order to access the CAS user e-mail address if available. The idea being that when a user logs in for the first time through CAS, the e-mail adress is automatically retrieved. But I don't know if it breaks compatibility with CAS 2 previously used

I can try to create a PR for this but I'm not sure about the testing part.

I can also provide the diff if that can help, there are not many lines of code