Add method to return raw Solr response headers

We use it in our SolrProbe to log the reported query execution time.

Cf. https://github.com/subhh/solrprobe/blob/master/src/main/php/SolrProbe.php#L104

module/VuFindSearch/src/VuFindSearch/Backend/Solr/Response/Json/RecordCollection.php

@@ -256,6 +256,16 @@ public function getCursorMark()
         return $this->response['nextCursorMark'] ?? '';
     }
 
+    /**
+     * Get response header.
+     *
+     * @return array
+     */
+    public function getResponseHeader ()
+    {
+        return $this->response['responseHeader'] ?? [];
+    }
+
     /**
      * Get raw Solr input parameters from the response.
      *