enh: add `maintain_order` to Expr.unique and Series.unique

[MarcoGorelli]

As requested here plotly/plotly.py#4790 (comment)

We already have maintain_order for DataFrame.unique, my concern is that for some libraries this may turn out to not be guaranteed (and for there to not be any option to guarantee it)

However, a workaround like

token = f'{s.name}_tmp'
s.to_frame().with_row_index(token).unique(col).sort(token).get_column(col)

should always be possible, were some library to stop guaranteeing it

It's not clear to me whether PyArrow makes any guarantees - we can always just use it for now, and if it stop making such guarantees, we use the workaround above apache/arrow#44663

[EdAbati]

To clarify, we just want this option for eager DataFrames/Expr, right?

[MarcoGorelli]

Yes, and for Series

When we get to the lazy-only layer, we should note that this option isn't guaranteed to be available

[raisadz]

I would like to work on it if that's ok