[FEA] Simplify creating aggregation requests

**Is your feature request related to a problem? Please describe.**
Currently creating the vector of aggregation requests can be fairly cumbersome due to the fact that nested vector elements must be modified in place.

**Describe the solution you'd like**
It should be possible to provide functions that encapsulate these multiple lines of code into a single function call that adds an aggregation request with the desired values and aggregations.

**Describe alternatives you've considered**
None

**Additional context**
An initial attempt was in https://github.com/rapidsai/cudf/pull/11476.