Add method to retrieve first indexed date.

module/VuFind/src/VuFind/RecordDriver/SolrDefault.php

@@ -135,6 +135,16 @@ class SolrDefault extends DefaultRecord
         parent::__construct($mainConfig, $recordConfig, $searchSettings);
     }
 
+    /**
+     * Get the date this record was first indexed (if set).
+     *
+     * @return string
+     */
+    public function getFirstIndexed()
+    {
+        return $this->fields['first_indexed'] ?? '';
+    }
+
     /**
      * Get highlighting details from the object.
      *