ENH/POC: EA.isin
- [x] closes #38340
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Needs tests; the MaskedArray.isin method is not currently reached.

cc @jorisvandenbossche
Closing in favor of #38379 @jbrockmendel thanks for starting this (and sorry for the slow response here), adding `isin` to the interface sounds like a good idea (eg also in the arrow-backed StringArray, we are going to want to override it).

Taking a look now at  #38379, but it might still be worth adding the general interface method in a separate PR (like this one), as it will need some more base extension tests + docs for that.

