Allow for other attribute sources than $_SERVER

[osnard]

Depending on the webserver configuration the Shibboleth attributes may not be available in $_SERVER, but for example in the return value of a getallheaders() call. I propose to encapsulate the attribute-extraction logic into configurable "provider" classes (e.g. $wgShibboleth_UserInfoProvider = "headers" or $wgShibboleth_UserInfoProvider = "server-super-global").

[northway]

You're welcome to create pull request. In the near future, I won't be able to work on this.

[osnard]

I have already started with the implementation but will probably wait until the repo has been moved to WMF gerrit.

[Shugabuga]

Any updates on this?

[northway]

I'll take a look at this on January.