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

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26580?src=pr&el=h1) Report
> Merging [#26580](https://codecov.io/gh/pandas-dev/pandas/pull/26580?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a60d1bd45a99519fad5024068db956e0aa1cc6a1?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26580/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26580?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26580      +/-   ##
==========================================
- Coverage   91.84%   91.84%   -0.01%     
==========================================
  Files         174      174              
  Lines       50643    50656      +13     
==========================================
+ Hits        46515    46524       +9     
- Misses       4128     4132       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.38% <100%> (ø)` | :arrow_up: |
| #single | `41.7% <7.14%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26580?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/26580/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9nZW5lcmljLnB5) | `89.27% <100%> (+0.18%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26580/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26580/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26580?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26580?src=pr&el=footer). Last update [a60d1bd...c008426](https://codecov.io/gh/pandas-dev/pandas/pull/26580?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26580?src=pr&el=h1) Report
> Merging [#26580](https://codecov.io/gh/pandas-dev/pandas/pull/26580?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0f3e8e8bd2118d56c7febe5d6081c5058cb22be7?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `93.75%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26580/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26580?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26580      +/-   ##
==========================================
- Coverage   91.71%   91.71%   -0.01%     
==========================================
  Files         178      178              
  Lines       50740    50755      +15     
==========================================
+ Hits        46538    46548      +10     
- Misses       4202     4207       +5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <93.75%> (ø)` | :arrow_up: |
| #single | `41.22% <6.25%> (-0.1%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26580?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/26580/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9nZW5lcmljLnB5) | `89.16% <93.75%> (+0.08%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26580/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26580/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.88% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26580?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26580?src=pr&el=footer). Last update [0f3e8e8...8fe39f1](https://codecov.io/gh/pandas-dev/pandas/pull/26580?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 can you merge master to ensure passing, otherwise lgtm. All green. thanks @TomAugspurger 