[ENH] concat operation on distributions

[fkiraly]

There should be a concat operation on distributions.

This will require:

• a "concatenated distribution" compositor skpro.concat, similar to pd.concat
• a dunder-like method in distributions to implement specific concat operations that allow to "flatten" concatenated distibutions of the same type

A direct consumer of this interface would be sktime.forecasting's predict_proba vectorization.