ENH: provide standard BaseIndexers in pandas.api.indexers
- [X] closes #33201
- [X] 1 test modified / 1 passed
- [X] passes `black pandas`
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

This PR adds a forward-looking `pandas.api.indexers.BaseIndexer` subclass implementation and exposes it to users as discussed in #33180 .

### Exposing other indexers

It is also possible to expose a backward-looking indexer simply by exposing `pandas.core.window.indexers.FixedWindowIndexer` through the API. Seems redundant to me as this is basically the same as supplying an integer as the window argument when creating a rolling object but it can be done for consistency. I'd be grateful for a discussion.

### Note on tests

The class is incorporated into `test_rolling_forward_window` in `test_base_indexer.py` and this serves as a test that the new indexer is working correctly.

### Notes on performance
I've tested the implementation and it shows comparable performance to `pandas.core.window.indexers.FixedWindowIndexer`. In my tests the new indexer was slightly faster, either by luck, or because I streamlined the backward-looking indexer a bit when making the forward-looking one.