[FEA] Add support for `cudf.DataFrame.aggregate`

**Is your feature request related to a problem? Please describe.**
We support `cudf.DataFrame.agg` but not `cudf.DataFrame.aggregate`.

```python
df = cudf.from_pandas(
pd.DataFrame(
[
[1, 2, 3],
[4, 5, 6],
[7, 8, 9]
],
columns=['A', 'B', 'C']
)
)

df.aggregate(['sum', 'min'])
```

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**Additional context**
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