step_rbert_features

I'm not sure yet if this should be inside RBERT or maybe integrated into [tidymodels/textrecipes](github.com/tidymodels/textrecipes), but we should make it easy to extract features from text in some sort of standard form using BERT checkpoints (within a recipes pipeline).

Start by determining whether this belongs in RBERT or tidymodels/textrecipes, then define the standard feature representation and how it fits into a recipes pipeline. Done means the integration location, BERT checkpoint handling, and expected extracted features are specified clearly enough to implement.

Written by the indexing model from the issue text.