MDEV-26938 Support descending indexes internally in InnoDB (server part)

* preserve DESC index property in the parser
* store it in the frm (only for HA_KEY_ALG_BTREE)
* read it from the frm
* show it in SHOW CREATE
* skip DESC indexes in opt_range.cc and opt_sum.cc
* ORDER BY test

This includes a fix of MDEV-27432.

mysql-test/suite/innodb/r/innodb-index.result

@@ -1969,7 +1969,7 @@ Table	Create Table
 t1	CREATE TABLE `t1` (
   `id` int(11) NOT NULL,
   PRIMARY KEY (`id`),
-  UNIQUE KEY `id` (`id`)
+  UNIQUE KEY `id` (`id` DESC)
 ) ENGINE=InnoDB DEFAULT CHARSET=latin1
 DROP TABLE t1;
 #