[EM] Enable prediction cache for GPU. (#10707)

- Use `UpdatePosition` for all nodes and skip `FinalizePosition` when external memory is used.
- Create `encode/decode` for node position, this is just as a refactor.
- Reuse code between update position and finalization.

tests/cpp/tree/hist/test_expand_entry.cc

@@ -1,10 +1,11 @@
 /**
- * Copyright 2023, XGBoost Contributors
+ * Copyright 2023-2024, XGBoost Contributors
  */
 #include <gtest/gtest.h>
 #include <xgboost/json.h>        // for Json
 #include <xgboost/tree_model.h>  // for RegTree
 
+#include "../../../../src/common/categorical.h"  // for CatBitField
 #include "../../../../src/tree/hist/expand_entry.h"
 
 namespace xgboost::tree {