Feat: Add precomputed fields

[hadim]

We need pydantic 2.x to be able to do that without an horrible hack :-) No rush and blocked by conda-forge/pydantic-feedstock#82

The idea is to add a few pre-computed fields whenever it makes sense.

A few examples for Dataset:

  repr_dict["n_datapoints"] = len(self)
  repr_dict["n_columns"] = len(self.table.columns)
  repr_dict["dataset_size_mb"] = 128