Add preview token to `preview_dataset()`

## Is your feature request related to a problem? Please describe.
We can't preview data pre-release using the `preview_dataset()` function

## Describe the solution you'd like
Copying the `preview_token` approach across from how it works for `query_dataset()`

## Describe alternatives you've considered
Not thought through, but assuming it's easy to just make available using existing code.

## Additional context
None