Use PHP attributes for route discovery

Problem/Motivation

Drupal should use modern PHP attributes for route discovery ala Symfony.

This will allow us to add an attribute to a method name and use that for routing information. For example:

  /**
   * The default 401 content.
   *
   * @return array
   *   A render array containing the message to display for 401 pages.
   */
  #[Route(
    path: '/system/401',
    name: 'system.401',
    requirements: ['_access' => 'TRUE'],
    defaults: ['_title' => 'Unauthorized']
  )]
  public function on401() {
    return [
      '#markup' => $this->t('Please log in to access this page.'),
    ];
  }

will replace the following entry in system.routing.yml

system.401:
  path: '/system/401'
  defaults:
    _controller: '\Drupal\system\Controller\Http4xxController:on401'
    _title: 'Unauthorized'
  requirements:
    _access: 'TRUE'

What are the advantages of doing this?

• The defaults._controller value is determined by the method you add the attribute to
• The route definition and the controller code live side by side
• No longer need route names - these can be automatically provided based on the class and method name
• Less difference between modern Symfony and Drupal

What are the disadvantages?

• The routing.yml file can give a good overview of what a module does and makes reviewing route access a bit simpler

Mitigations of the disadvantages:

• Currently the code is scanning all the classes in a module's src/Controllers directory for attributed methods - therefore it's not that difficult to scan this code and you benefit from the controller and route definition being together.
• We should and can build CLI tools to expose all the routes provided by a module. This would be good not just for routes determined by attributes but also dynamic routes

Proposed resolution

• Remove yaml discovery from the RouteBuilder into it's own service
• Add a new static routing event for the yaml discovery to listen to
• Add a new service for PHP attribute discovery and listen to the same event. This reflects on all classes in a module's src/Controllers directory to find routes

Remaining tasks

• Add more test coverage
• Decide which of the Symfony route features to keep in - env, localized_paths, priority, _locale, _canonical_route stuff... not sure what to do with this.

User interface changes

None

API changes

TBD

Data model changes

None

Release notes snippet

@todo