Add annotations

https://nbviewer.org/github/JetBrains/lets-plot-kotlin/blob/master/docs/examples/jupyter-notebooks/f-4.5.0/bar_annotations.ipynb
https://nbviewer.org/github/JetBrains/lets-plot-kotlin/blob/master/docs/examples/jupyter-notebooks/f-4.2.0/annotations_for_pie.ipynb

Start by comparing the two linked notebooks, bar_annotations.ipynb and annotations_for_pie.ipynb, with the current Kandy plotting API. Identify which annotation behaviors those examples require and determine how they should be exposed in Kandy. Done means the referenced annotation examples work in the project’s notebook environment, though the issue does not name tests or implementation files.

Written by the indexing model from the issue text.