Support skipping individual columns in DbCopier (use DB defaults)

[nafg]

Add a way to tell DbCopier to skip specific columns, bypassing column coverage validation and omitting them from the INSERT column list entirely (so the database uses DEFAULT values).

JDBC PreparedStatement can't pass DEFAULT as a parameter — the only way to get defaults is to omit the column from the INSERT. This would require a new OutputColumn variant or a separate skip mechanism (e.g., skippedColumns: Map[String, Set[String]] on DbCopier).