Support for tuple keys in add_columns function

[Hibbins]

I prefer to use add_columns over add_column. But when I want to grab individual cell values or using similar functions I run into some issues and I haven't been able to find a robust way of dealing with it.

This addition the the add_columns function should solve any issues of getting a proper column key, as the user would be allowed to set the keys themselves, similar to the option you have with add_column. Personally I miss this feature on the add_columns function. I do not know if that is shared, but I figured I would propose a solution at least.

I opened a discussion here: #5922

Checklist

• Docstrings on all new or modified functions / classes
• Updated documentation (assuming auto generation on textual.textualize.io)
• Updated CHANGELOG.md (where appropriate)