BUG: Allow named aggregation with Resampler.agg
- [x] closes #32803 
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

Thanks @dicristina for the PR. can you add a release note doc/source/whatsnew/v1.4.0.rst thanks @dicristina very nice!