Add groupby::make_aggregation_request and groupby::make_scan_request factory functions

[ttnghia]

TBA

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[vyasr]

@ttnghia do you recall if this is still something we need to do?

[ttnghia]

Yes, this is something that can simplify/shorten the code when we want to perform an aggregation. It is purely for convenient purposes.
For example, currently we want to write like this:

  std::vector<cudf::groupby::aggregation_request> requests;
  requests.emplace_back(cudf::groupby::aggregation_request());
  requests[0].values = values;
  requests[0].aggregations.emplace_back(cudf::make_argmin_aggregation<cudf::groupby_aggregation>());

  auto result = gb_obj.aggregate(requests,...);

We can do better by:

  auto const requests = make_aggregation_request(values, cudf::make_argmin_aggregation<cudf::groupby_aggregation>());
  auto result = gb_obj.aggregate(requests,...);