ASV: add benchmarks for groupby cython aggregations
I noticed that we currently don't really have a groupby benchmark that specifically targets the cython aggregations. We have of course benchmarks that call those, but eg `GroupByMethods` is setup in such a way that it's mostly benchmarking the factorization and post-processing (also useful of course), while eg for sum only 7% is spent in the actual `groupby_add` algorithm.

So therefore this PR is adding an additional benchmark more targetted at the cython aggregation (where 30-50% of the time is spent in the actual aggregation function, so we will more easily catch regressions/improvements there)

For now I only added "float64". I could also add "float32", but not sure how useful that is (since they are all using fused types, it's the same implementation for both dtypes.