DEPR: .str accessor with object dtype

[jbrockmendel]

• closes DEPR: Deprecate .str accessor on object-dtype. #29710 (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations 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.

There are two cases we need an answer for before merging this. One, as I just posted in #29710

looking at doing this, the main problem is .decode. For everything else we can tell users to do obj.astype("str").str, but if you have bytes to start out with, that doesn't work. Unless we assume pyarrow is available and then can tell users to do .astype("bytes[pyarrow]").str.decode

The other is when you can't cast to "str" bc you have funky entries, like in test_to_hdf_errors data = ["\ud800foo"].