QAN explain doesn't rewrite UDPATE queries

EXPLAIN does not work for non-SELECT statements if the pmm user has only limited privileges

EXPLAIN DELETE/INSERT/REPLACE/UPDATE requires the user to have the corresponding privilege, there is an open MySQL bug: https://bugs.mysql.com/bug.php?id=73258

the preferred fix would be to rewrite those statements to a SELECT, similar to the PMM 1 qan-agent (https://github.com/percona/qan-agent/blob/master/query/plugin/mysql/explain/transform.go)