ENH: Support fold argument in Timestamp.replace
- [x] closes #25017
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Since `Timestamp` has its own `replace` method, I think we can still introduce this while still supporting PY3.5 (`datetime.replace` gained the `fold` argument in 3.6) while it mimics the functionality in PY3.6