Create cache context service for AMP library warnings config

In #2688545: Remove ampNodeViewController in favor of amp_entity_view_alter(), we are removing the ampNodeViewController in favor of changing the AMP view mode in amp_entity_view_alter.

However, that means we can no longer set cache keys, as those cannot be changed in alter hooks.

In reviewing caching information at https://www.drupal.org/developing/api/8/cache/contexts, it appears what would be a better option would be to create a cache context service for this configuration setting, and add that context amp_entity_view_alter. We already have a cache context to vary based on whether or not the warnfix query parameter is present. So the service only needs to focus on varying based on something like so (except with dependency injection):

  // Get the Config Factory service.
  /** @var ConfigFactoryInterface $config_factory */
  $config_factory = \Drupal::service('config.factory');

  // First check the config if library warnings are on
  $amp_config = $config_factory->get('amp.settings');
  if ($amp_config->get('amp_library_warnings_display')) {
    return true;
  }