feat: add onlyOnType field for SCFacet

src/protocol/search/facet.ts

@@ -13,6 +13,7 @@
  * this program. If not, see <https://www.gnu.org/licenses/>.
  */
 import {SCThingsField} from '../../meta';
+import {SCThingType} from '../../things/abstract/thing';
 
 /**
  * A search facet
@@ -27,6 +28,11 @@ export interface SCFacet {
    * Field of the aggregation
    */
   field: SCThingsField;
+
+  /**
+   * Type of the aggregation
+   */
+  onlyOnType?: SCThingType;
 }
 
 /**