ENH: Add HalfYear offsets
- [ ] closes #60928
- [ ] closes #22362
- [ ] [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).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

@rhshadrach I was able to add the half-year offset classes without too much added complexity. However, I haven't enabled them to be used as periods - that will take more code changes and tests. Do you recommend concluding this here or go all the way to supporting half-year periods? cc: @MarcoGorelli  @snitish - I think holding off on periods makes sense. This is already a sizable PR. Thanks again @snitish 