design the option for user to control ordering for window union rows and original rows that have same ts manully

**Describe the feature you'd like**

#1776 defines the default ordering, is orginal row before union rows.
It may still necessary to define a new SQL option in query or a new system variable to control whether original rows should be first or last.

**Additional context**

Add any other context or screenshots about the feature request here.

The issue references #1776 for default ordering. Examine the window union implementation in the codebase, likely in SQL parsing or query execution modules. Determine where ordering logic is applied and how a new SQL option or system variable could be integrated. Review existing system variables and SQL option patterns in OpenMLDB to understand the extension mechanism.

Written by the indexing model from the issue text.