Making selectors public

[TheooJ]

In #1116, we discussed allowing users to use skrub selectors for the cols parameter. Selecting columns is very frequent in skrub transformers, and they might be useful for users. They are currently private, if we want to render them public, we need to:

• Document them. Their docstrings are already quite exhaustive but we need a dedicated example / reference in the doc
• Define for which parameters / transformers we allow selectors
• Support them in those cases (Joiner, AggJoiner, MultiAggJoiner, SelectCols, DropCols, TableVectorizer, etc)

[GaelVaroquaux]

I would like us to move forward with the recipe API first. I know that it is a big thing, but I think that it will help us understand the role and position of the selectors.

[TheooJ]

Fair enough, let's see how this develops then :)