ENH: Let Resampler objects have a pipe method
- [x] closes #17905
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Currently, calls to ``df.resample(....).pipe(...)`` are converted to ``df.resample(....).mean().pipe(...)`` and a warning is emitted (see #17905).

This PR solves this by moving the ``pipe`` method from the ``GroupBy`` class to the ``_GroupBy`` class. As ``_GroupBy`` is a common parent class of both ``GroupBy`` and ``Resampler``, the ``pipe`` method is now available for ``Resampler`` too.

See also #17871.