ENH: enabled 'on' keyword for groupby(..).resample()
 - [x] closes #15021 
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15326?src=pr&el=h1) Report
> Merging [#15326](https://codecov.io/gh/pandas-dev/pandas/pull/15326?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/34cdfa48881118a6327fe0e599fb41467ef6ffcc?src=pr&el=desc) will **increase** coverage by `<.01%`.


```diff
@@            Coverage Diff             @@
##           master   #15326      +/-   ##
==========================================
+ Coverage   86.32%   86.33%   +<.01%     
==========================================
  Files         141      141              
  Lines       51163    51164       +1     
==========================================
+ Hits        44169    44170       +1     
  Misses       6994     6994
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15326?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/compare/34cdfa48881118a6327fe0e599fb41467ef6ffcc...9fc3b4fa45112c1eb69197bb4d9c553546ea5a78?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `95.15% <100%> (ø)` | :white_check_mark: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15326?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/15326?src=pr&el=footer). Last update [34cdfa4...9fc3b4f](https://codecov.io/gh/pandas-dev/pandas/compare/34cdfa48881118a6327fe0e599fb41467ef6ffcc...9fc3b4fa45112c1eb69197bb4d9c553546ea5a78?el=footer&src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). thanks @verhalenn 

changed slightly to put in a more logical place. 

we have had this dichotomy between ``.groupby`` and ``.resample`` where the former uses ``.key`` and latter ``.on``. We had a discussion about this to normalize that, but didn't take any action.

@chrisaycock @jorisvandenbossche 

worth deprecating ``key`` in favor of ``on`` in ``pd.Grouper``? or just leave it (I happen to like ``on`` better as we use this in ``.merge`` as well). > worth deprecating `key` in favor of `on` in `pd.Grouper`?

Makes sense to me. Consistent parameter names makes things easy. I would personally just leave this. 
The current 'sounds' also OK: "resample on a column", group by a key" (not "group by on") ok, so split opinion! going to make an issue in any event.