Allow benchmarks per column

[MarcusKlik]

For example:

synthetic_bench() %>%
  bench_tables(generator, column_mode = "single column") %>%
  bench_streamers(rds_streamer, fst_streamer, parguet_streamer, feather_streamer) %>%
  bench_rows(1e7, 5e7) %>%
  bench_compression(50, 80) %>%
  compute()

Parameter column_mode could specify the exact mode:

• 'all': just benchmark the complete table
• 'single': benchmark each column separately.
• 'per type': split the table horizontally per column type
• numerical vector: construct a new table (per type?) from existing columns with the specified number of columns