✨ [ClickBench] Remove redundant projections

[joocer]

We are diagnosing performance issues with the ClickBench benchmark and identified this inefficiency. It is unlikely to be material whilst on a single thread executor, but when multithreaded, the cost of the additional call will increase.

This plan has two redundant projections, after the scan we only have the URL column so the two projections aren't doing anything

└─ EXIT
   └─ PROJECT (URL)
      └─ SUBQUERY AS a
         └─ LIMIT (1000000)
            └─ PROJECT (URL)
               └─ FILTER (URL LIKE '%google%')
                  └─ SCAN (hits) [URL]

[joocer]

In trying to address this a bug was found in the projection pushdown strategy - it changes the type of the columns attribute in the Scan, SubQuery and Union operators to be lists of FlatColumns rather than LogicalColumns.

Fixing this is likely to have knock-on impacts to these operators and potentially to other optimization strategies.