PERF: add shortcut to Timedelta constructor
WIP until #30676 gets merged.
- [ ] second half of #30543
- [X] tests added 1 / passed 1
- [X] passes `black pandas`
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

This implements a shortcut in the Timedelta constructor to cut down on processing if Timedelta is passed. We still need to check if any other args are passed. Then, if a Timedelta with no other kwargs was passed, we just return that same Timedelta.
A test is added to check that the Timedelta is still the same object.