ENH: Named aggregation in SeriesGroupBy.agg
```python
In [4]:    animals = pd.DataFrame({'kind': ['cat', 'dog', 'cat', 'dog'],
   ...:                            'height': [9.1, 6.0, 9.5, 34.0],
   ...:                            'weight': [7.9, 7.5, 9.9, 198.0]})
   ...:    animals.groupby("kind").height.agg(max_height='max')
Out[4]:
      max_height
kind
cat          9.5
dog         34.0
```

Closes #26512

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
