ENH: Support kurtosis (kurt) in DataFrameGroupBy and SeriesGroupBy
- [x] closes #40139 
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

DataFrameGroupBy and SeriesGroupBy currently support mean, std and skew (the first 3 moments) but not kurtosis (the 4th moment). This change addresses that. I implemented kurtosis in cython in similar fashion to skewness. I've verified that the output of this function matches that of DataFrame.kurt().
Thanks for the review @rhshadrach. 

- Addressed your comments
- Added test case for skipna=False (by default it's true)
- Added test case for float64[pyarrow] (we already have one for float64)
- Added test case for constant data. Note that the result here is 0.0, consistent with `DataFrame.kurt()` and `Series.kurt()` Apologies @snitish - this fell off my radar. Should be able to get to it today or tomorrow. Thank you @rhshadrach! Thanks @snitish 