ENH: Support skipna parameter in GroupBy mean and sum
- [ ] partially addresses #15675
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/v3.0.0.rst` file if fixing a bug or adding a new feature.

This PR would only partially close #15675. There are other reductions that still need to support `skipna`. Once approved, I plan to address the remaining.
Thanks @snitish  Thanks @mroeschke for approving. Since there are some leftover reduction operations in the original list that still need to support `skipna`, could you please reopen the original issue #15675 ? Sure thing done.