New TwigAllowed method attribute

The access of Twig templates to object methods can now allowed by the TwigAllowed attribute.
Example:

class AttributeAllowTestClass {

  #[\Drupal\Core\Template\Attribute\TwigAllowed]
  public function allowed(): string {
    return __METHOD__;
  }

  public function notAllowed(): string {
    return __METHOD__;
  }

  // Still allowed, but soon deprecated.
  public function getFoo(): string {
    return 'foo';
  }

}

Note that

• The legacy behavior of allowing methods with magic prefixes is still working, but may be deprecated in the future.
• The legacy method of adjusting said patterns via settings.php is still working, but discouraged.