Exclude disabled JSON API resources

Confirming the patch works with 2.x branch of jsonapi and openapi_ui_redoc:8.x-1.0-rc2. Since jsonapi_extras doesn't work yet with jsonapi 2.x we've implemented some custom code to only allow some entities for now.

<?php
/**
 * Implements hook_entity_type_alter().
 */
function my_module_entity_type_alter(array &$entity_types) {
  $allow = [
    'comment',
    'file',
    'node',
    'user',
  ];

  /** @var $entity_types \Drupal\Core\Entity\EntityTypeInterface[] */
  foreach ($entity_types as $entity_name => $entity_type) {
    /** @var $entity_type \Drupal\Core\Entity\EntityTypeInterface */
    if (in_array($entity_name, $allow, TRUE) !== TRUE) {
      $entity_type->set('internal', TRUE);
    }
  }
}

Patch in #2 works against openapi 8.x-1.0-beta2, using jsonapi 8.x-2.4 and jsonapi_extras 8.x-3.5. It no longer shows the disabled resources.

I was working on a related patch for excluding disabled fields in schemata (#2882269: Support for JSON API's ResourceType::getPublicName() and ResourceType::isFieldEnabled()). I think that it would be best to implement this there, since schemata is used in jsonapi (contrib) to test validity of schema, and it makes sense to pull in schema without alterations in order to provide a consistent export of schema.

I was working on the related schemata patch last week during sprints at DrupalCon Seattle. I will finish up the updated patch and work this functionality into it, since its most relevant there.

I think this may be a duplicate of #2882269: Support for JSON API's ResourceType::getPublicName() and ResourceType::isFieldEnabled(), but leaving it open for now since this one has a patch.

Patch passed tests and looks good. It's been committed and pushed.