[FEATURE] specify number of trees for shap

[pseudotensor]

Problem: the approximate method can still be slow for many trees
catboost version: master
Operating System: ubuntu 18.04
CPU: i9
GPU: RTX2080

Would be good to be able to specify how many trees to use for shapley. The model.predict and prediction_type versions allow this. lgbm/xgb allow this.

[Evgueni-Petrov-aka-espetrov]

Many thanks for the idea!