Make certificate fields conditional

Problem/Motivation

Certificate related fields should be only visible if the field "Use certificate for login (use_cert)" is set to "Use certificate"

Proposed resolution

Make $form['plugin_settings']['oauth2_client']['cert_file_path'] and $form['plugin_settings']['oauth2_client']['cert_file'] fields conditional with '#states' settings:

'#states' => [
  'visible' => [
    ':input[name="use_cert"]' => ['value' => '1'],
  ],
],