ENH/POC: EA.isin

[jbrockmendel]

• closes ENH: implement fast isin() for nullable dtypes #38340
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

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

cc @jorisvandenbossche

[jreback]

+1 on this. can you add some generic tests (on the EAs themselves). obviously we cover on Series via dispatching but should still have some tests.

[jbrockmendel]

Closing in favor of #38379

[jorisvandenbossche]

@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.