recipes:::strings2factors should be restricted by the role

[dajmcdon]

Feature

prep.recipe() calls recipes:::strings2factors() to convert character columns to factors (when true). It would be better if this operated, not on all columns but only on those with certain roles (likely just predictors + outcomes).

Context: I'm working on an extension that would accommodate panel data. This involves keeping around a key column which needs to remain a character regardless of the decisions for predictor columns.

I'm happy to do a PR for this if it would be helpful. I think it requires only minor changes, but hopefully would cause few downstream issues.

[EmilHvitfeldt]

I think that sounds reasonable. If you feel like it, feel free to start the PR 😄

[dajmcdon]

Closed by #1376