Allow Keyword aggregation in SeriesGroupBy

[TomAugspurger]

Followup to #26399

In [1]: df = pd.DataFrame({'A': [1, 1, 1, 2, 2], 
   ...:                      'B': range(5), 
   ...:                      'C': range(5)})                                    

In [2]: df                                     
Out[2]: 
   A  B  C
0  1  0  0
1  1  1  1
2  1  2  2
3  2  3  3
4  2  4  4

In [3]: df.groupby('A').B.agg(foo='count')                                                        
Out[3]: 
   foo
A     
1    3
2    2

#26399 (review)