Allow using "default" #captcha_type for the captcha form element

Problem/Motivation

When adding a captcha to a form by using the captcha form element, you can't simply use "default" as #captcha_type, while it would be nice to be able to do so. Currently you have to retrieve the default challenge from captcha like this:

    $default_challenge = \Drupal::service('config.manager')
      ->getConfigFactory()
      ->get('captcha.settings')
      ->get('default_challenge');

    $form['captcha'] = [
      '#type' => 'captcha',
      '#captcha_type' => $default_challenge,
    ];

For example see the famous Webform module, which has to use this code.

Steps to reproduce

Proposed resolution

Allow using

$form['captcha'] = [
      '#type' => 'captcha',
      '#captcha_type' => 'default',
    ];

(or another placeholder value) and replace "default" by the default captcha type in the captcha module, before processing the element. That way other modules don't have to know about CAPTCHA modules internals.

Remaining tasks

User interface changes

API changes

Data model changes