Add OIDC plugin activation control

Add the ability for an admin to activate the OIDC plugin in a way that
does not require updating the ProjectConfiguration class file, or using
the plugin admin interface.

If the empty file 'activate-oidc-plugin' is present in the root of the
AtoM installation, the OIDC plugin will be enabled.

At this point in Symfony's startup, in ProjectConfiguration::setup()
config files like app.yml are not yet loaded which is why the presence
of an OS file is being used.

config/ProjectConfiguration.class.php

@@ -48,6 +48,12 @@ public function setup()
             'sfPluginAdminPlugin',
         ];
 
+        // Check if the OIDC plugin should be enabled.
+        $filePath = 'activate-oidc-plugin';
+        if (file_exists($filePath) && filesize($filePath) === 0) {
+            $plugins[] = 'arOidcPlugin';
+        }
+
         $this->enablePlugins($plugins);
 
         $this->dispatcher->connect(