Improve the table diff sample output in CLI

[izeigerman]

Currently the printed output is too wide and unreadable even for tables with a handful of columns, since we include columns from both the source and the target table as part of the same row.

Here are some ideas on how to make the output more digestible:

• Print individual column pairs. This way the width will be bounded to 2 columns that are being compared + join keys.
• Let user select which columns should be included in the sample.