[FEA] `aggregation` should be more opaque

**Is your feature request related to a problem? Please describe.**

The `aggregation` class is used for specifying details of what kind of aggregation a user wants from APIs like `groupby`. User's should not need to interact with this class other than using the `make_*_aggregation` APIs to get an `aggregation` object.

Originally this type was opaque, but if I remember correctly, that broke Cython. So we had to make it non-opaque. However, this has lead to issues of users using methods of `aggregation` that they should not.

**Describe the solution you'd like**
`aggregation` should be made more opaque. The PIMPL pattern is probably the right call here such that the type's definition is still visible to appease Cython, but it's members aren't available.