Rework reduction case statement as dispatch_type_and_aggregation (#20078)

Reworks the switch/case statement in reductions.cpp to use `dispatch_type_and_aggregation` instead.
This allows the specific functors to indicate valid aggregation/type support, reduce behavior when no data is given, as well as the reduction function itself. This makes it easier to update existing aggregations and support newer ones more reliably. It also allows for a reduction-specific `is_valid_aggregation()` API by keeping making the valid checks a compile-time feature.

Authors:
  - David Wendt (https://github.com/davidwendt)

Approvers:
  - Bradley Dice (https://github.com/bdice)
  - Nghia Truong (https://github.com/ttnghia)

URL: #20078

Author: davidwendt

cpp/include/cudf/reduction.hpp

@@ -255,6 +255,20 @@ std::pair<std::unique_ptr<scalar>, std::unique_ptr<scalar>> minmax(
   rmm::cuda_stream_view stream      = cudf::get_default_stream(),
   rmm::device_async_resource_ref mr = cudf::get_current_device_resource_ref());
 
+/**
+ * @brief Reduction namespace
+ */
+namespace reduction {
+/**
+ * @brief Indicate if a reduction is supported for a source datatype.
+ *
+ * @param source The source data type.
+ * @param kind The reduction aggregation.
+ * @returns true if the reduction is supported.
+ */
+bool is_valid_aggregation(data_type source, aggregation::Kind kind);
+}  // namespace reduction
+
 /** @} */  // end of group
 
 }  // namespace CUDF_EXPORT cudf