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ARROW-11591: [C++][Compute] Grouped aggregation

This patch adds basic building blocks for grouped aggregation:

- `Grouper` for producing integer arrays encoding group id from batches of keys
- `HashAggregateKernel` for consuming batches of arguments and group ids, updating internal sums/counts/...

For testing purposes, a one-shot grouped aggregation function is provided:
```c++
std::shared_ptr<arrow::Array> needs_sum = ...;
std::shared_ptr<arrow::Array> needs_min_max = ...;
std::shared_ptr<arrow::Array> key_0 = ...;
std::shared_ptr<arrow::Array> key_1 = ...;

ARROW_ASSIGN_OR_RAISE(arrow::Datum out,
  arrow::compute::internal::GroupBy({
    needs_sum,
    needs_min_max,
  }, {
    key_0,
    key_1,
  }, {
    {"sum", nullptr},  // first argument will be summed
    {"min_max", &min_max_options},  // second argument's extrema will be found
}));

// Unpack struct array result (a four-field array)
auto out_array = out.array_as<StructArray>();
std::shared_ptr<arrow::Array> sums = out_array->field(0);
std::shared_ptr<arrow::Array> mins_and_maxes = out_array->field(1);
std::shared_ptr<arrow::Array> group_key_0 = out_array->field(2);
std::shared_ptr<arrow::Array> group_key_1 = out_array->field(3);
```

Closes #9621 from bkietz/groupby1

Lead-authored-by: Benjamin Kietzman <bengilgit@gmail.com>
Co-authored-by: michalursa <michal@ursacomputing.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>