Add OpenID Connect (#5351)

* Add OIDC

* Update documentation.

* Update apache conf adding IfModule

* Use IfDefine for OIDC in apache conf

* Fix non-oidc support

* Fix typing

* Use IfDefine to enable OIDC

* Add OIDC support to all dockerfiles

* Re add apache Require option

* Fixes and documentation

* A few more fixes

* A bit more doc

* Change type of environment variable

* Update readme

* Correct apache config for OIDC support.

* Fix README formatting

* Update oidc control path

* Fix oidc endpoint being cached

* A bit more review

* Simplify ExpiresActive

* Add session refresh and improve caching

* Allow more different setups

* A bit more documentation

* A bit more readme

---------

Co-authored-by: Aaron Schif <aschif@netdevgroup.com>
Co-authored-by: Alexandre Alapetite <alexandre@alapetite.fr>
Co-authored-by: maTh <math-home@web.de>

docs/en/admins/09_AccessControl.md

@@ -37,6 +37,10 @@ WARNING: FreshRSS will trust any IP configured in the `trusted_sources` option,
 Not using authentication on your server is dangerous, as anyone with access to your server would be able to make changes as an admin.
 It is never advisable to not use any form of authentication, but **never** chose this option on a server that is able to be accessed outside of your home network.
 
+## OpenID Connect
+
+* See [dedicated section](16_OpenID-Connect.md).
+
 ## Hints
 
 You can switch your authentication method at any time by editing the `./data/config.php` file, on the line that begins `'auth_type'`.