Fix WebInstaller-Break

Set wgAuth only, if GoogleLoginAuth exists and move this setup
into wgExtensionFunctions.
Fix type in Documentation of GoogleLoginAuth

Bug: 69564
Change-Id: I6e18e61cc721ca069e3cbbce9787d429344b7474
(cherry picked from commit f7bb3f99ee6e033a3db27c019d33c4fd873e34df)