ExtensionArray.Round method and tests
- [x] closes #49387
- [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.

This adds abstract round method in ExtensionOpsMixin to tell EA authors to implement round, and provides a round test.

Any pointers on how to get mypy to accept two function signitures for round? Datetimelikes get overridden:
```
    def round(self, decimals: int = 0, *args, **kwargs) -> Self:
```
gets overridden by
```
    def round(
        self,
        freq,
        ambiguous: TimeAmbiguous = "raise",
        nonexistent: TimeNonexistent = "raise",
    ) -> Self:
``` If the typing overload decorator doesn't work I think in this case it's OK to ignore the mypy failures since it's a known differnece.

https://docs.python.org/3/library/typing.html#overload this is ready for review again Let's do this! poke @jbrockmendel  This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this. Couple small comments, otherwise LGTM.  Thanks for your patience.