Allow to Shadow Virtual Column _table in Merge Engine

Why it matters

The feature allows users to define a real column named _table in Merge engine tables, which overrides the default virtual _table column that normally indicates the source table of each row. This solves conflicts when a real _table column is needed, improving schema flexibility and enabling custom use cases.

How to use it

To use this feature, simply create or alter a table with the Merge engine by adding a column named _table. This real column will shadow the existing virtual _table column without requiring additional configuration.