BUG: GH29461 strftime() with nanoseconds for Timestamp
- [x] closes #29461 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

The new method displays %f to the full precision, including trailing zeroes. I could add a check that only displays nanoseconds if they are non-zero.