Add a "hidden" property to the column

[jcabot]

This property allows the dataset owner to discard a column for the bot generation.

Not to be confused with the discussions on a column is part of a specific resultset or not.

Here we want to give the option to completely hide the column as if it was not in the CSV at all. E.g. it could just be because the column contains sensitive data that the owner doesn't want to expose it

By default, the property should be set to false (i.e. all columns are exposed by default)

[mgv99]

Columns are deleted in the csv copied to the generated bot (as if they didn't exist). This way we don't modify the original data but create a bot that doesn't have access to some columns