ENH: EA._cast_pointwise_result

[jbrockmendel]

• closes REF: make _cast_pointwise_result an EA method #59895 (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.

Still have a bunch of pyarrow tests involving duration/timestamp dtypes failing. Also need to update/remove the test files' _cast_pointwise_result methods.

xref #56430, could close that with a little effort.

I suspect a bunch of "pyarrow dtype retention" tests are solved by this, will update as I check. Nope!

[jbrockmendel]

The pyarrow duration stuff is caused by an upstream issue apache/arrow#40620

[jbrockmendel]

@rhshadrach i think you had a recent issue/pr involving retaining nullable dtypes in a .map?

[rhshadrach]

Really great improvement here. It seems to me there are two potential situations we might find ourselves in: (a) take the dtype of self into account or (b) don't take the dtype of self into account. I highlighted one specific example below where I think this might go awry. The base implementation is doing (b) whereas the subclasses are doing various degrees of (a). Understand that isn't being introduced here, but I think the long term goal is to make this more consistent?

For now, do we want to setup the framework to separate these two cases out somehow - perhaps an argument to _cast_pointwise_result?

I see now that this is just preserving the dtype when possible. I think we don't need two different cases as I first imagined.

[rhshadrach]

lgtm

[mroeschke]

Thanks @jbrockmendel