Add Spider status variable to count usage of GROUP BY handler

The Spider storage engine has status variables for counting the number of times direct UPDATE and direct DELETE. https://mariadb.com/kb/en/spider-status-variables/#spider_direct_delete

It would be convenient that there is a status variable to count the number of times the direct SELECT is performed (i.e., GROUP BY handler is used). Currently, a user can distinguish direct SELECT and non-direct SELECT by the result of EXPLAIN, but this is not consistent and makes it difficult to collect statistics about the direct SELECT.

From the consistency viewpoint, the name of the status variable would be Spider_direct_select or Spider_group_by_handler.