Allow drivers to customize DDLInvoker

Drivers currently can't override the functionality provided by `DDLInvoker`. For a specific driver for a currently unsupported database I write, the driver has to issue some SQL statements that don't work with `PreparedStatement`s. The driver would need to provide a custom `DDLInvoker` to execute all DDL queries with regular `Statement`s.

This commit adds a `createDDLInvoker` factory method similar to `createInsertInvoker`, which is then used by the implicit method `ddlToDDLInvoker`.


Regards,
  Gerolf