feat(plotting): add sampling param

I've been meaning to do this for a while now. Plotting functions now have a num_series param (and seed) to sample some series (by default, 10). In this way, users don't have to do the filtering themselves.

Don't know if this implementation is optimal. We are still collecting all entities.

I also would like to implement a "rank_by" to plot images in order. Adding another parameter is the fastest way but I am not in love with it: too much complexity.

I am leaving this here, as food for thought: I am tempted by the OOP approach:

y = functime.PanelDataFrame.from_polars(...) # basically a LazyFrame with some type checks and stores the "state" AKA entity, time and value cols

y.rank(by=..., how="best").plot(n_series=10)

functime-org / functime

feat(plotting): add sampling param #158