ENH: Implement Keyword Aggregation for DataFrame.agg and Series.agg
- [x] closes #26513, related to #28380 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This PR basically deals with three issue:
1. implement keyword aggregation for DataFrame.agg
2.  implement keyword aggregation for Series.agg
