adding allow_experimental_variant_type settings

Author: mzitnik

client-v2/src/main/java/com/clickhouse/client/api/internal/SettingsConverter.java

@@ -94,7 +94,8 @@ public static String escape(String value) {
             ClickHouseClientOption.MAX_RESULT_ROWS.getKey(),
             "extremes",
             "role",
-            "describe_include_subcolumns"
+            "describe_include_subcolumns",
+            "allow_experimental_variant_type"
     )));
 
     public static Map<String, ClickHouseOption> createMapOfRequestOptions() {