Allow specifying `meta` data on JSON:API objects

From the JSON:API spec: https://jsonapi.org/format/#document-meta

Where specified, a meta member can be used to include non-standard meta-information. The value of each meta member MUST be an object (a “meta object”).

Any members MAY be specified within meta objects.

Handling of metadata in the JSON:API normalizers already exists.

RelationshipNormalizer

    return CacheableNormalization::aggregate([
      'data' => $this->serializer->normalize($object->getData(), $format, $context),
      'links' => $this->serializer->normalize($object->getLinks(), $format, $context)->omitIfEmpty(),
      'meta' => CacheableNormalization::permanent($object->getMeta())->omitIfEmpty(),
    ]);

ResourceIdentifierNormalizer

    if ($object->getMeta()) {
      $normalization['meta'] = $this->serializer->normalize($object->getMeta(), $format, $context);
    }

The problem is there's no way to add meta information to these objects. See ResourceObjectNormalizer::serializeField

      if ($field instanceof EntityReferenceFieldItemListInterface) {
        // Build the relationship object based on the entity reference and
        // normalize that object instead.
        assert(!empty($context['resource_object']) && $context['resource_object'] instanceof ResourceObject);
        $resource_object = $context['resource_object'];
        $relationship = Relationship::createFromEntityReferenceField($resource_object, $field);
        $normalized_field = $this->serializer->normalize($relationship, $format, $context);
      }

createFromEntityReferenceField takes a meta and links parameter. But there's no way to add information.

This is useful for building a headless application on Drupal to provide context about the information.