text: Title: [tabular] Add target encoding preprocessing option
  Body: Add target encoding preprocessing option. Likely best as a toggle for models, such as `"ag.target_encoding": True`.
  
  Examples of target encoding getting performance improvements:
  
  1. Kaggle Playground Series: https://www.kaggle.com/code/theoviel/explaining-and-accelerating-target-encoding
  2. Nested K-Fold: https://github.com/rapidsai/deeplearning/blob/main/RecSys2020Tutorial/03_3_TargetEncoding.ipynb
  3. category_encoders package: https://contrib.scikit-learn.org/category_encoders/index.html