ENH/POC/WIP: EA-backed Index
Still TODO
- [x] <s>288</s>15 not-yet-passing tests (with --skip-slow --skip-db)
- [ ] flesh out tests/extension/base/ea_index.py
- [x] <s>deprecation cycle</s> (decided we can probably just change)
- [ ] optimize EAIndexEngine/NullableIndexEngine once #43870 is resolved
- [x] EA.putmask implementation ATM is just a stub (and needs tests)
- [ ] whatsnew

cc @jorisvandenbossche @TomAugspurger 

xref https://github.com/pandas-dev/pandas/issues/39133