[C++] Create many-column scanner microbenchmarks

When developing we often assume schemas are cheap and small only to find out later that we create easily avoided bottlenecks for users that have very large schemas. We should create some micro-benchmarks for the scanner that verify we get roughly the same performance, in data-bytes-per-second, with many-columns as we do with few-columns (note, that we probably suffer in rows-per-second since we are loading more columns and thus more data).

This might also be a good time to create similar benchmarks for dataset discovery.