validate column: Character columns are skipped for min/max validations

Character columns are skipped for min/max validations, for example:
```
data-validation -v validate column -sc ora_conn -tc pg_connl -tbls myschema.customers \
-min='first_name' -max='first_name'
...
03/02/2023 04:38:33 PM-INFO: Skipping min on first_name due to data type: string[non-nullable]
03/02/2023 04:38:33 PM-INFO: Skipping max on first_name due to data type: string[non-nullable]
```

I believe this means the only aggregated validation we can do is count. I don't think this is adequate for proving a migration.

Presumably string columns are skipped because of potential problems around localization and multi-byte characters.

Would it be worth having an option stating we trust the source/target data and want to validate the specified string columns?