Simple saml review

I made the review in the form a merge request since the changes are somehow difficult to address via suggestions.

The main things I changed:

• The SimpleSAMLPHP library should and must be included via composer
• The code that actually authenticates against the saml idp was duplicated. I'm not really sure where it should better be - in the constructor or verifyUsername(). Other plugins tend to use verifyUsername().
• The rest is basically formal stuff regarding compliance to PSR-12 or using the features PHP8 provides.

I hope, I didn't break anything. If this MR will be merged into the original TIC, I will request another review from someone else and then the auth plugin can hopefully get included in Stud.IP 6.0.