[FEA] Implement `observed` parameter for `groupby`

Doing `DataFrame.groupby(..., observed=False)` currently produces:

```
NotImplementedError: observed parameter is not yet implemented
```

**Additional context**
Not sure if this is already being tracked somewhere, but the missing argument is requiring some extra code in https://github.com/rapidsai/cudf/pull/14805 that I'd like to remove later, and I'd like to have an issue to link to in the code.