ENH: Support ExtensionArray in Groupby
```python
In [1]: import pandas as pd

In [2]: from cyberpandas import IPArray

In [3]: df = pd.DataFrame({"A": IPArray([0, 0, 1, 2, 2]), "B": [1, 5, 1, 1, 3]})

In [4]: df
Out[4]:
         A  B
0  0.0.0.0  1
1  0.0.0.0  5
2  0.0.0.1  1
3  0.0.0.2  1
4  0.0.0.2  3

In [5]: df.groupby("A").B.mean()
Out[5]:
A
0.0.0.1    1
0.0.0.2    2
Name: B, dtype: int64
```

Note that right now `Out[5].index` just just an `Index` with object dtype. In the future, we could tie an Index type to an ExtensionArray type, and ensure that the extension type propagates through.