SimpleSamlPHP auth plugin

This is a merge request related to this TIC that I created to add support for SimpleSamlPHP: #4365.

I wasn't sure whether I should edit the original TIC or write everything here, so below is all the information needed to understand the code and set up the test environment. The issue contains more general information. If I did anything wrong, please let me know, and I will fix it.

Files

• StudipAuthSimpleSamlPHP.php - contains the plugin
• logout.php - modified public/logout.php for support of SingleLogout (I just reused code that was used for CAS logout)
• config_defaults.inc.php - configuration examples (same as for other auth plugins)

prerequisites

• SimpleSamlPHP installed on the machine in its default dirrectory (can be changed in plugin code)
  • use other session than php_session as this would interfere with Stud.IP session

Testing enviroment

On this branch I have prepared testing enviroment with mocksaml as an IdP. All you have to do to set it up is start docker-compose.yml (docker-compose up) and Stud.IP with SimpleSamlPHP support will be available at http://localhost:8032. Then you can login with mocksaml IdP by using 'federated login' link.