Switch from generating coordinate tables to generating annotation polygons

[alexkrz]

For slide-level classification tasks, it turns out to be more generic if we store polygons for tissue regions instead of fixed patch locations. This structure would then also be compatible with the expected input to the slide_tools Repo: https://github.com/DBO-DKFZ/slide_tools

For the data structure of the Polygon it would be best to adopt the GeoJson style from QuPath.
More information on how QuPath exports annotations can be found here: https://qupath.readthedocs.io/en/stable/docs/advanced/exporting_annotations.html