Sometimes the pushed down TopN(tikv, tiflash) can keep the order expression to save cpu or network

[winoros]

Enhancement

The above is one example.

Currently, we keep the output columns unchanged when there's topn pushed down to tikv/tiflash.
But sometimes the order by expression may reduce the size of the column data transferred by the network significantly.
And another example is that the expression's evaluation cost is rather high. In that time we can also keep the expression, not to calculate it twice on tidb and tiflash/tikv.

[EricZequan]

Implement: #58208