[ENH] dummy supervised regressor with polars support

[julian-fong]

Implement the DummyProbaRegressor but with complete end to end support in skpro.

Some current limitations:

fit inside DummyProbaRegressor uses skpro.distributions which only supports pandas dataframes - needs a workaround

predict_proba also uses skpro.distributions - leading to the same issue, will need a workaround as well

@fkiraly any suggestions on how to implement?