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