Allowing CatBoost for `model_tau` input to Explainer class

[portchester1989]

Is your feature request related to a problem? Please describe.
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Even if I would like to use CatBoost for model_tau for Explainer class, I cannot bc there's not exposure of feature_importances_. And in case we have

Describe the solution you'd like
modify check_condition and default_importance methods under Explainer class

Describe alternatives you've considered
custom implementation of Explainer class on my end

Additional context