Make datatype callback handling more flexible

• Datatype callbacks are used to convert a field value to a specific format for machine processing. We accept arguments to this callback in addition to the value. For instance, instead of hard-coding UserComments UserInteraction into RDF module's Schema.org Data Converter callback, we pass it in as an argument with the key 'interaction_type'.
• You may use any of the UserInteraction types as the 'interaction_type' argument to the datatype_callback callable method or function 'Drupal\rdf\SchemaOrgDataConverter::interactionCount'.

In Drupal 7 this was defined in hook_rdf_mapping(). In Drupal 8, this goes in the YAML for a content type. For instance, a created at field and a comment count:

Before

/**
 * Implements hook_rdf_mapping().
 */
function node_rdf_mapping() {
  return array(
    array(
// ...
        'created' => array(
          'predicates' => array('dc:date', 'dc:created'),
          'datatype' => 'xsd:dateTime',
          'callback' => 'date_iso8601',
        ),
// ...
        'comment_count' => array(
          'predicates' => array('sioc:num_replies'),
          'datatype' => 'xsd:integer',
        ),
// ...
      ),
    ),
  );
}

After

  created:
    properties:
      - 'schema:dateCreated'
    datatype_callback:
      callable: 'date_iso8601'

and

  comment_count:
    properties:
      - 'schema:interactionCount'
    datatype_callback:
      callable: 'Drupal\rdf\SchemaOrgDataConverter::interactionCount'
      arguments:
        interaction_type: 'UserComments'