Provide insights on the columns with stats collected

[MrPowers]

Some Delta implementations only collect stats on certain columns.

Here are some APIs that will help users debug their questions:

• what columns are stats being collected for
• for a given column, are stats being collected in all the files?
• what columns have stats being collected for some files, but not other files

The get_add_actions API should make this possible.

[robertkossendey]

Great idea, do we want to return the result as an object, so the user can work with the output programatically, or just print it?

[MrPowers]

@robertkossendey - I generally prefer outputting results programmatically, so users can use the functions in programs. There are some exceptions, but I think computer-readable output (that's also human friendly) is usually the best goal.

[robertkossendey]

@MrPowers you can assign me here, I'd like to work on this.