[FEA] Serialization of libcudf classes and exposing implementation details

**Is your feature request related to a problem? Please describe.**
For multi-gpu polars, we will require serializing certain data in Python to be passed between Dask workers, for example `aggregation`s. In https://github.com/rapidsai/cudf/pull/17469 I've proposed a way to do that, however, that proposal requires certain implementation details from [`aggreation.hpp`](https://github.com/rapidsai/cudf/blob/branch-25.02/cpp/include/cudf/detail/aggregation/aggregation.hpp),
more specifically classes derived from `aggregation`, such as [`std_var_aggregation`](https://github.com/rapidsai/cudf/blob/0e01dbd2d8b2a048964d27d1fd9d9c3bf6540166/cpp/include/cudf/detail/aggregation/aggregation.hpp#L384-L413). @vyasr has pointed out to the fact that those details are not exposed to pylibcudf and would be best if it continues like that.

**Describe the solution you'd like**
The solution proposed in https://github.com/rapidsai/cudf/pull/17469 seems to be the lowest hanging fruit, but as described above may not be considered optimal for several reasons.

**Describe alternatives you've considered**
Exposing attributes of the classes publicly may be an alternative, but that would incur in a different set of potential issues.

I'm not familiar with most of the design and options available in libcudf, so it's likely core developers will see other potentially better alternatives.