Implement HOST_UDF aggregation for groupby

[ttnghia]

This implements HOST_UDF aggregation, allowing to execute a host-side user-defined function (UDF) through libcudf aggregation framework.

• A host-side function can be an arbitrarily independent function running on the host machine. It may or may not call other device kernels depending on its implementation.
• Such user-defined function must follow the libcudf provided interface (cudf::host_udf_base). The interface provides the ability to fully interact with libcudf aggregation framework.
• Since it is implemented on the user application side, it has a very high degree of freedom to perform arbitrary operations to satisfy the user's need.

Partially contributes to #16633.

---

Usage

1. Define a functor deriving from cudf::host_udf_base and implement the required virtual functions declared in that base struct. For example:

struct my_aggregation : cudf::host_udf_base {
   ...
};

2. Create an instance of libcudf HOST_UDF aggregation which is constructed from an instance of the functor defined above. For example:

auto agg = cudf::make_host_udf_aggregation<cudf::groupby_aggregation>(
    std::make_unique<my_aggregation>());

3. Perform aggregation operation on the created instance.

[PointKernel]

Looks great. Just a few nits and questions on how to improve the documentation.

[PointKernel]

Two small questions otherwise looks great

[PointKernel]

LGTM ship it :) (oh my, sorry for the typo. corrected)

[res-life]

LGTM for the Java part.

[vyasr]

CMake approval (didn't look at the C++).

[ttnghia]

/merge