feat(optimizer): SExpr visitor (#17768)

* add SExpr visitor unit test

* add SExprVisitor example

Author: BohuTANG

src/query/service/tests/it/sql/planner/optimizer/ir/expr/mod.rs

@@ -0,0 +1,15 @@
+// Copyright 2021 Datafuse Labs
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+mod visitor;