feat(python): Make groupby rolling/dynamic iterable

[stinodego]

Resolves #5786

Question: while working on this, I noticed that groupby_rolling defaults to closed=left, while groupby_dynamic defaults to closed=right. Why is this? It would make sense to me to have both default to closed=left.

[zundertj]

In finance applications, where typically you have a backward (in time) looking window, you would want closed="right" to be the default imo. See for example Pandas doing the same with rolling_mean.

[ritchie46]

For groupby rolling I believe it was most intuitive as then each row is the first element of its respective group. In groupby dynamic we don't have this direct row 1:1 mapping.

[ritchie46]

Nice one. I can imagine this being useful for ergonomics on smallish data.

[drivenow]

@zundertj @ritchie46 Do you think it's suitable to use rs-polars in high frequency market data indicators calculation? which features streaming Calculation and extreme low latency.