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.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18940?src=pr&el=h1) Report
> Merging [#18940](https://codecov.io/gh/pandas-dev/pandas/pull/18940?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ee9c7e9b19a1fa3c4499d10885dec72f1d651832?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18940/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18940?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18940      +/-   ##
==========================================
+ Coverage   91.59%   91.59%   +<.01%     
==========================================
  Files         150      150              
  Lines       48959    48964       +5     
==========================================
+ Hits        44845    44850       +5     
  Misses       4114     4114
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.96% <100%> (ø)` | :arrow_up: |
| #single | `41.13% <75%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18940?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/resample.py](https://codecov.io/gh/pandas-dev/pandas/pull/18940/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzYW1wbGUucHk=) | `96.36% <100%> (+0.01%)` | :arrow_up: |
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/18940/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `92.08% <100%> (ø)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18940/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.95% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18940?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18940?src=pr&el=footer). Last update [ee9c7e9...30f91be](https://codecov.io/gh/pandas-dev/pandas/pull/18940?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 nice @topper-123 thanks!