feat(polars): allow user to specify "engine" kwarg

[deepyaman]

Description of changes

Modeled after support for "streaming" kwarg, but I have a couple questions:

1. I don't see any tests for "streaming", so I didn't add any for "engine"; is this fine?
2. I didn't see any attempt to support older Polars versions that didn't accept "streaming", so I didn't try to do that for "engine", either. However, Polars is technically stable now; should we not force people to get latest?

[cpcloud]

We don't have tests for streaming, but we have a blog post that uses it, so it has had coverage.

It would be good to verify this behavior does something in the test suite. It can be a smoke test.

[jcrist]

Maybe to avoid requiring a GPU for support we could do a mocktest just to ensure we're plumbing the engine kwarg through properly? Then if things fail we at least know it's not on our end.

[cpcloud]

Yeah, using mocker.spy would be sufficient IMO.

[deepyaman]

Yep, sounds good, I was planning to mock it (and maybe actually go ahead and do the same for making sure streaming gets passed, too).