feat: join_where?

[MarcoGorelli]

Might join_where be in-scope? I think that for Polars / DuckDB / PySpark / Ibis it should be doable, and very nice-to-have. I don't think pandas / pyarrow have equivalents, but maybe that's OK - for most of our API, it's other libraries that raise NotImplementedError, maybe it's OK if in some cases it's pandas / pyarrow that do so 😄

@camriddell this seems like it may be up your street, do you have any thoughts?