RED-8622: implemented add entity functionality.

src/lib/listing/services/entities.service.ts

@@ -61,6 +61,12 @@ export class EntitiesService<
         return this._entityDeleted$.pipe(filter(entity => entity.id === entityId));
     }
 
+    addEntity(entity: Class): void {
+        if (!this.find(entity.id)) {
+            this._all$.next([...this.all, entity]);
+        }
+    }
+
     setEntities(entities: Class[]): void {
         const changedEntities: Class[] = [];
         const deletedEntities = this.all.filter(oldEntity => !entities.find(newEntity => newEntity.id === oldEntity.id));