Add 'class' to migration configuration entity schema

It's an edge case, but a different migration plugin class from the default can be specified in migration configuration. A core example from d6_profile_values.yml:

class: Drupal\user\Plugin\migrate\ProfileValues

class is not defined in the configuration schema, thus attempts to export and import migrations using it (as in migrate-upgrade's --configure-only) drop the class configuration. We need to add it to the schema.