PERF: add shortcut to Timestamp constructor
- [X] closes #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 Timestamp constructor to cut down on processing if Timestamp is passed. We still need to check if the timezone was passed correctly. Then, if a Timestamp was passed, and there is no timezone, we just return that same Timestamp.
A test is added to check that the Timestamp is still the same object.

PR for timedelta to be added once I confirm that this is the approach we want to go with.
