CLN: aggregation.transform
- [x] closes #36330
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Followup to #35964 

- Moved whatsnew to reshaping
- Use is_list_like/is_dict_like in aggregate.transform
- Broke out dict-like and str/callable computations to own functions to split up the transform function
- Added/refined typing
/azp run <samp>
Azure Pipelines successfully started running 1 pipeline(s).<br>

</samp> /azp run <samp>
Azure Pipelines successfully started running 1 pipeline(s).<br>

</samp> @jreback Tests added and pass; responses to your questions are above. Thanks for reviewing @ivanovmg. I don't necessarily disagree with your suggestions, but I think they are outside the scope of this PR. @jreback The diff on this is getting to be too complex (and honestly started that way). I've opened #36618 as a precursor. @jreback - This is ready for another review. Failure is unrelated. thanks @rhshadrach very nice