Adding expandEntityBaseFields() method.

[grasmash]

This pull request allows expansion of base fields on entities. In Drupal 8, base fields use the same field types as user-defined fields. E.g., a custom entity may define an entity reference field as a base field. Without this PR, there is no mechanism for expanding such fields.

Example usage:

      /** @var Drupal8 $d8_core */
      $d8_core = $this->getDriver()->getCore();
      $entity = (object) $row;
      $base_fields = [
        'licensed_entity',
        'user_id',
      ];
      $d8_core->expandEntityBaseFields('license', $entity, $base_fields);
      $d8_core->entityCreate('license', $entity);

This would expand the licensed_entity and user_id base fields using the EntityReferenceHandler class.

This pull request also documents a couple of undocumented parameters and fixes a bug that prevented user entity values from being expanded on entity reference fields.

[grasmash]

This new method is public. expandEntityFields() cannot be used for custom entities because it is protected.

[jhedstrom]

Sorry for the delay. This is merged now. Thanks!