[Function] add BE bitmap function bitmap_max (#6942)

Support bitmap_max.

docs/en/sql-reference/sql-functions/bitmap-functions/bitmap_max.md

@@ -0,0 +1,55 @@
+---
+{
+    "title": "bitmap_max",
+    "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.
+-->
+
+# bitmap_max
+## description
+### Syntax
+
+`BIGINT BITMAP_MAX(BITMAP input)`
+
+Calculate and return the max values of a bitmap.
+
+## example
+
+```
+mysql> select bitmap_max(bitmap_from_string('')) value;
++-------+
+| value |
++-------+
+|  NULL |
++-------+
+
+mysql> select bitmap_max(bitmap_from_string('1,9999999999')) value;
++------------+
+| value      |
++------------+
+| 9999999999 |
++------------+
+```
+
+## keyword
+
+    BITMAP_MAX,BITMAP