Provide main entity metadata in drupalSettings and code component utils package

[#3565754]

This MR:

• Introduces a new key called mainEntity in drupalSettings that provides the entity's UUID, entityTypeId, and bundle.
• Expand function getPageData() from drupal-utils which returns the above mainEntity
• main entity can either be:
  • mainEntity: null // on non-entity route (inside code editor, home page, /user/login)
  • mainEntity: {bundle: 'article', entityTypeId: 'node', uuid: '8d32b7ea-9c19-4cd3-a008-9a374ba6e876'} (on an entity route)

Example usage:

On article 2 and the code component code filters out the current entity (article 2) from the related article using drupal-jsonapi-paramsand filter by the current entity uuid.

import { getPageData } from 'drupal-canvas';
import { JsonApiClient } from 'drupal-canvas';
import { DrupalJsonApiParams } from 'drupal-jsonapi-params';
import useSWR from 'swr'

const client = new JsonApiClient();

function RelatedArticles({ mainEntity }) {
  const { bundle, entityTypeId, uuid } = mainEntity;
  const { data, error, isLoading } = useSWR(
  [
    'node--article',
    {
      queryString: new DrupalJsonApiParams()
        .addFilter('id', uuid, '<>') // Exclude current article by uuid.
        .getQueryString(),
    },
  ],
  ([type, options]) => client.getCollection(type, options),
  );

   return (...);
}

// Wrapper component to check for mainEntity existence and type before calling a hook since
// hooks cannot be called conditionally (React rules of hooks).
function RelatedArticlesWrapper() {
  const { mainEntity } = getPageData();

  // Return early if there is no mainEntity, or it is not an article node.
   if (
     !mainEntity ||
     mainEntity.entityTypeId !== 'node' ||
     mainEntity.bundle !== 'article'
   ) {
     return null;
  }
  return <RelatedArticles mainEntity={mainEntity} />;
}

export default RelatedArticlesWrapper;