Direct multi-table update/delete interface

Currently direct_update_rows() / direct_delete_rows() handler calls are limited to single-table statements. Multi-table UPDATE/DELETE bypasses this path and is fully executed by the SQL layer, preventing storage engines from optimizing execution.
Extend the API to allow engines to handle multi-table DML pushdown.
Example:

UPDATE orders o

  JOIN customers c ON o.customer_id = c.id

SET o.status = 'active'

WHERE c.region = 'EU';