Implement categorical data support for SHAP. (#7053)

* Add CPU implementation.
* Update GPUTreeSHAP.
* Add GPU implementation by defining custom split condition.

tests/cpp/predictor/test_predictor.h

@@ -68,6 +68,8 @@ void TestPredictionWithLesserFeatures(std::string preditor_name);
 void TestCategoricalPrediction(std::string name);
 
 void TestCategoricalPredictLeaf(StringView name);
+
+void TestIterationRange(std::string name);
 }  // namespace xgboost
 
 #endif  // XGBOOST_TEST_PREDICTOR_H_