Add support for DROP TEMPORARY TABLE IF EXISTS

[lukaseder]

MySQL supports the IF EXISTS clause specifically also for TEMPORARY tables, see https://dev.mysql.com/doc/refman/8.0/en/drop-table.html

As we've seen in a related issue, the TEMPORARY keyword does make a difference in MySQL, so the combination of keywords is worth supporting, even if TEMPORARY tables can also be dropped without the TEMPORARY keyword.