[FEA] cudf.Series.agg support

**Is your feature request related to a problem? Please describe.**
working with `import cudf as pd`

**Describe the solution you'd like**
`cudf.Series.agg` matching https://pandas.pydata.org/docs/reference/api/pandas.Series.agg.html

```
>>> import cudf, pandas
>>> cudf.__version__
'23.02.00a+310.g58e0fde346'
>>> pandas.__version__
'1.5.3'
>>> cudf.Series([1, 1, 42, 1984]).agg('mean')
Traceback (most recent call last):
File "", line 1, in
AttributeError: 'Series' object has no attribute 'agg'
>>> pandas.Series([1, 1, 42, 1984]).agg('mean')
507.0
```