upcasting converters replaced with ParamConverter Service

The old 'converters' option no longer exists. It's been replaced by 'parameters'

options:
  converters:
    foo: 'user'

New way:

options:
  parameters:
    foo:
      type: 'entity:user'

In a Route Subscriber, the options array looks like this:

          array(
            'parameters' => array(
              'entity' => array(
                'type' => 'entity:' . $entity_type,
              ),
            ),
          )

ParamConverters need to implement ParamConverterInterface and be tagged paramconverter in services.yml file

class LanguageConverter implements ParamConverterInterface {

  /**
   * {@inheritdoc}
   */
  public function convert($value, $definition, $name, array $defaults) {
    $languages = language_list(Language::STATE_ALL);
    return isset($languages[$value]) ? $languages[$value] : NULL;
  }

  /**
   * {@inheritdoc}
   */
  public function applies($definition, $name, Route $route) {
    if (!empty($definition['type']) && $definition['type'] == 'language') {
      return TRUE;
    }
    return FALSE;
  }

}

services:
  language_converter:
    class: Drupal\language\LanguageConverter
    tags:
      - { name: paramconverter }