Implement can_be_manually_set()

This suppresses user upload warnings and allows oidc to be selected as
a default auth method if none are included in the csv.

auth.php

@@ -73,6 +73,15 @@ class auth_plugin_oidc extends \auth_plugin_base {
         $this->config = $this->loginflow->config;
     }
 
+    /**
+     * Returns true if plugin can be manually set.
+     *
+     * @return bool
+     */
+    function can_be_manually_set() {
+        return true;
+    }
+
     /**
      * Returns a list of potential IdPs that this authentication plugin supports. Used to provide links on the login page.
      *