Does not work with config_override_warn

Problem/Motivation

When overriding the configuration for one of the clients, config_override_warn does not detect that it is overridden.
That's because OpenIDConnectSettingsForm::getEditableConfigNames() does not return the client configurations.

Steps to reproduce

1. Install openid_connect and config_override_warn.
2. Add something like this to your settings.php file:

   $config['openid_connect.settings.generic']['settings']['authorization_endpoint'] = 'https://example.com/oauth2/authorize';

3. Then browse to /admin/config/services/openid-connect
4. No warning is displayed

Proposed resolution

OpenIDConnectSettingsForm::getEditableConfigNames() should return every configuration handled by this form.