BUG: Support timespec argument in Timestamp.isoformat() (#26131)
- [x] closes #26131
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

I have just tried to support the same behaviour as `datetime.datetime()`, without adding the additional `"nanoseconds"` option, but this could be handled as well.