[Function] Support utf-8 encoding in instr, locate, locate_pos, lpad, rpad (#3638)

Support utf-8 encoding for string function `instr`, `locate`, `locate_pos`, `lpad`, `rpad`
and add unit test for them

docs/zh-CN/sql-reference/sql-functions/string-functions/lpad.md

@@ -31,7 +31,7 @@ under the License.
 `VARCHAR lpad(VARCHAR str, INT len, VARCHAR pad)`
 
 
-返回 str 中长度为 len（从首字母开始算起）的字符串。如果 len 大于 str 的长度，则在 str 的前面不断补充 pad 字符，直到该字符串的长度达到 len 为止。如果 len 小于 str 的长度，该函数相当于截断 str 字符串，只返回长度为 len 的字符串。
+返回 str 中长度为 len（从首字母开始算起）的字符串。如果 len 大于 str 的长度，则在 str 的前面不断补充 pad 字符，直到该字符串的长度达到 len 为止。如果 len 小于 str 的长度，该函数相当于截断 str 字符串，只返回长度为 len 的字符串。len 指的是字符长度而不是字节长度。
 
 ## example
 

docs/zh-CN/sql-reference/sql-functions/string-functions/rpad.md

@@ -0,0 +1,54 @@
+---
+{
+    "title": "rpad",
+    "language": "zh-CN"
+}
+---
+
+<!-- 
+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.
+-->
+
+# rpad
+## description
+### Syntax
+
+`VARCHAR rpad(VARCHAR str, INT len, VARCHAR pad)`
+
+
+返回 str 中长度为 len（从首字母开始算起）的字符串。如果 len 大于 str 的长度，则在 str 的后面不断补充 pad 字符，直到该字符串的长度达到 len 为止。如果 len 小于 str 的长度，该函数相当于截断 str 字符串，只返回长度为 len 的字符串。len 指的是字符长度而不是字节长度。
+
+## example
+
+```
+mysql> SELECT rpad("hi", 5, "xy");
++---------------------+
+| rpad('hi', 5, 'xy') |
++---------------------+
+| hixyx               |
++---------------------+
+
+mysql> SELECT rpad("hi", 1, "xy");
++---------------------+
+| rpad('hi', 1, 'xy') |
++---------------------+
+| h                   |
++---------------------+
+```
+##keyword
+RPAD