ENH : First attempt to add sparse_pivot
- [ ] Resolves #14493 
- [ ] tests added / passed
- [ ] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

``` python
sparse_series = pivot_sparse(index=df['a'], columns=df['b'], values=df['c'])

```

this would need to be an actual sparse pivot implementation. converting to dense then pivotting doesn't really work here. If you have some thoughts on that, pls reopen / comment. hmm.. i didn't convert to dense though...

the idea is just transforming the index instead of data/values.   it's like keep the data in a series and update the new index which is after pivot


