feat: add DROP pipe operator support

[jayendra13]

Which issue does this PR close?

• Closes Support SQL pipe operator #14660 (partial — adds DROP pipe operator only)

Rationale for this change

The DROP pipe operator (|> DROP x, y) is part of the BigQuery pipe syntax tracked in #14660. It removes columns from the output, equivalent to SELECT * EXCEPT(x, y).

What changes are included in this PR?

• Added PipeOperator::Drop match arm in query.rs that reuses existing SELECT * EXCEPT infrastructure (WildcardOptions with ExceptSelectItem)
• Added SLT tests covering single column drop, multi-column drop, chaining with other pipe operators, and error case for non-existent columns
• Added DROP to the pipe operator documentation in select.md

Are these changes tested?

Yes, four sqllogictest cases added in pipe_operator.slt:

• Drop two columns
• Drop single column
• Chained with WHERE
• Error on non-existent column

Are there any user-facing changes?

Yes, users can now use |> DROP column1, column2 in pipe operator queries (BigQuery dialect).

[simonvandel]

Very cool, thanks. The changes look good to me.

I'm not a committer, so someone else needs to do the final review.