_normalize_keyword_aggregation` named agg helper function should be made public
Slightly related to #28380 

Currently an issue in Dask https://github.com/dask/dask/issues/5294 for implementing Named Aggregation (introduced in pandas 0.25.0) is open. To implement this it needs to use `_normalize_keyword_aggregation` and `_is_multi_agg_with_relabel`. 

Making it public would be useful for frameworks like [dask](https://github.com/dask/dask) [mars](https://github.com/mars-project/mars)

cc: @TomAugspurger 