[Bug] Fix string functions not support multibyte string (#3345)

Let string functions support utf8 encoding

docs/en/sql-reference/sql-functions/string-functions/char_length.md

@@ -0,0 +1,55 @@
+---
+{
+    "title": "CHAR_LENGTH",
+    "language": "en"
+}
+---
+
+<!-- 
+Licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements.  See the NOTICE file
+distributed with this work for additional information
+regarding copyright ownership.  The ASF licenses this file
+to you 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.
+-->
+
+# char_length
+## Description
+### Syntax
+
+'INT char_length (VARCHAR str)'
+
+
+Returns the length of the string and the number of characters returned for multi-byte characters. For example, five two-byte width words return a length of 5, only utf8 encodeing is support at current version.
+
+## example
+
+
+```
+mysql> select char_length("abc");
++--------------------+
+| char_length('abc') |
++--------------------+
+|                  3 |
++--------------------+
+
+mysql> select char_length("中国");
++------------------- ---+
+| char_length('中国')   |
++-----------------------+
+|                     2 |
++-----------------------+
+```
+## keyword
+CHAR_LENGTH