Enhancement: Add `get_feature_importance()` to `AutoML`

[pplonski]

It would be very useful to have a unified method like:

feat_imp = automl.get_feature_importance()

to easily retrieve feature importance from the AutoML object.

Currently, there is no direct method available, and users need to access it manually per model.

Suggestion:
Add a get_feature_importance() method that aggregates or returns feature importance for the best model (or all models).