[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/en/sql-reference/sql-functions/string-functions/lpad.md

@@ -28,10 +28,10 @@ under the License.
 ## Description
 ### Syntax
 
-'VARCHAR lpad (VARCHAR str., INT len, VARCHAR pad)'
+'VARCHAR lpad (VARCHAR str, INT len, VARCHAR pad)'
 
 
-Returns a string of length len in str, starting with the initials. If len is longer than str, pad characters are added to STR until the length of the string reaches len. If len is less than str's length, the function is equivalent to truncating STR strings and returning only strings of len's length.
+Returns a string of length len in str, starting with the initials. If len is longer than str, pad characters are added to STR until the length of the string reaches len. If len is less than str's length, the function is equivalent to truncating STR strings and returning only strings of len's length. The len is character length not the bye size.
 
 ## example
 

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

@@ -0,0 +1,54 @@
+---
+{
+    "title": "rpad",
+    "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.
+-->
+
+# rpad
+## Description
+### Syntax
+
+'VARCHAR rpad (VARCHAR str, INT len, VARCHAR pad)'
+
+
+Returns a string of length len in str, starting with the initials. If len is longer than str, pad characters are added to the right of STR until the length of the string reaches len. If len is less than str's length, the function is equivalent to truncating STR strings and returning only strings of len's length. The len is character length not the bye size.
+
+## 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