Add "renderedForm" property to webform that returns the HTML

Example GraphQL query:

{
  webformById(webform_id: "contact") {
    title
    renderedForm
  }
}

Closes #3443423

src/Plugin/GraphQL/Fields/Webform/WebformRenderedForm.php

+<?php
+
+namespace Drupal\graphql_webform\Plugin\GraphQL\Fields\Webform;
+
+use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Drupal\Core\Render\RendererInterface;
+use Drupal\graphql\GraphQL\Execution\ResolveContext;
+use Drupal\graphql\Plugin\GraphQL\Fields\FieldPluginBase;
+use GraphQL\Type\Definition\ResolveInfo;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Provides a field handler which renders an webform into HTML.
+ *
+ * @GraphQLField(
+ *   secure = true,
+ *   parents = {"Webform"},
+ *   id = "webform_rendered_form",
+ *   name = "renderedForm",
+ *   type = "String",
+ * )
+ */
+class WebformRenderedForm extends FieldPluginBase implements ContainerFactoryPluginInterface {
+
+  /**
+   * The renderer service.
+   *
+   * @var \Drupal\Core\Render\RendererInterface
+   */
+  protected $renderer;
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, array $configuration, $pluginId, $pluginDefinition) {
+    return new static(
+      $configuration,
+      $pluginId,
+      $pluginDefinition,
+      $container->get('renderer')
+    );
+  }
+
+  /**
+   * WebformRenderedForm constructor.
+   *
+   * @param array $configuration
+   *   The plugin configuration array.
+   * @param string $pluginId
+   *   The plugin id.
+   * @param mixed $pluginDefinition
+   *   The plugin definition.
+   * @param \Drupal\Core\Render\RendererInterface $renderer
+   *   The renderer service.
+   */
+  public function __construct(
+    array $configuration,
+    $pluginId,
+    $pluginDefinition,
+    RendererInterface $renderer,
+  ) {
+    parent::__construct($configuration, $pluginId, $pluginDefinition);
+    $this->renderer = $renderer;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function resolveValues($value, array $args, ResolveContext $context, ResolveInfo $info) {
+
+    $builder = \Drupal::service('entity_type.manager')->getViewBuilder('webform');
+    $view = $builder->view($value);
+
+    yield $this->renderer->render($view);
+  }
+
+}