BUG: groupby.var with ArrowDtype(pa.decimal128)
- [x] closes #54627 (Replace xxxx with the GitHub issue number)
- [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
- [ ] 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.


I don't think modifying `_arrow_dtype_mapping` is completely correct because I'm not sure we want to do this conversion all the time.

Looks like the solution of converting to masked is not generic enough depending on the agg function. Probably needs a more native solution