Optimize Hex and add related Doc (#6697)

I tested hex in a 1000w times for loop with random numbers，
old hex avg time cost is 4.92 s，optimize hex avg time cost is 0.46 s which faster nearly 10x.

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

@@ -0,0 +1,63 @@
+---
+{
+    "title": "unhex",
+    "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.
+-->
+
+# unhex
+## description
+### Syntax
+
+`VARCHAR unhex(VARCHAR str)`
+
+Enter a string, if the length of the string is 0 or an odd number, an empty string is returned;
+If the string contains characters other than `[0-9], [a-z], [A-Z]`, an empty string is returned;
+In other cases, every two characters are a group of characters converted into hexadecimal, and then spliced into a string for output.
+
+
+## example
+
+```
+mysql> select unhex('@');
++------------+
+| unhex('@') |
++------------+
+|            |
++------------+
+
+mysql> select unhex('41');
++-------------+
+| unhex('41') |
++-------------+
+| A           |
++-------------+
+
+mysql> select unhex('4142');
++---------------+
+| unhex('4142') |
++---------------+
+| AB            |
++---------------+
+```
+## keyword
+UNHEX