audit log for DDL operations

[lmatz]

I imagine one approach is to put it into the log
another one is to put it into a system table that can be queried.
I feel the latter one is better as the log is managed. For on-premium users, I guess it is always a pain to bring another log management system which may bring more difficulties for us to debug when providing support.

DML operations could be too many and they are less critical.
DDL only sounds more reasonable.