[C++][Dataset] Support Count function without projections in ORC to avoid loading all columns

For ORC support in dataset, when execute count query without projections, just like "select count from table", it will load all columns. Because orc lib code is that https://github.com/apache/orc/blob/22828f79a526069d9629719c9476b7addad91ae6/c%2B%2B/src/Reader.cc#L120-L144.

Arrow side can improve it like parquet in dataset.