ENH/POC: ExtensionIndex for arbitrary EAs
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Currently 4 tests failing locally bc lib.infer_dtype raises on DecimalArray and JsonArray (xref #37367)

Not _remotely_ enough testing.

cc @jorisvandenbossche 