[ENH] adapter to scikit-survival, all distributional survival regressors interfaced

[fkiraly]

Adds an adapter to scikit-survival, exposing array-like survival functions as Empirical distributions in predict_proba.

Adds all models from scikit-survival which are capable of full distributional predictions:

• CoxPhSurvivalAnalysis
• CoxNetSurvivalAnalysis
• SurvivalTree
• GradientBoostingSurvivalAnalysis
• ComponentwiseGradientBoostingSurvivalAnalysis
• RandomSurvivalForest
• ExtraSurvivalTrees

The remaining models, e.g., SVM, are not capable of distributional predictions.