format of `add_tailor(method)`

[simonpcouch]

The way that method ought to be specified might require revisiting. Currently:

https://github.com/tidymodels/workflows/blob/9e6ca982f8cd381cf4601cceb64f5ad323737476/R/post-action-tailor.R#L25-L31

This forefronts pieces of the API (classes of rsample objects) that are not user-facing for most users at the moment. At the same time, few users will need to use this argument (can be set automatically in a principled way if fitting in tune, see #232) and its current default is likely okay for most on-label uses (besides time-based initial splits).

Related: new_action_tailor() currently does not type check method, pending a decision on the format of that argument:

https://github.com/tidymodels/workflows/blob/9e6ca982f8cd381cf4601cceb64f5ad323737476/R/post-action-tailor.R#L213

We should also include an example supplying method once we've decided on its format.