Specifying datatype when adding a column to a dataset.
#self-assign
### Feature request

There should be a way to specify the datatype of a column in `datasets.add_column()`. 

### Motivation

To specify a custom datatype, we have to use `datasets.add_column()` followed by `datasets.cast_column()` which is slow for large datasets. Another workaround is to pass a `numpy.array()` of desired type to the `datasets.add_column()` function.

IMO this functionality should be natively supported.

https://discuss.huggingface.co/t/add-column-with-a-particular-type-in-datasets/95674



### Your contribution

I can submit a PR for this.