Allow for right-open cumulative operations

[t-ded]

Description

Problem description:
I have encountered multiple times the necessity to compute some cum_* op in a way that would not consider the value of the current row. I have not found an issue addressing this.

Usecase:
For a very simplistic usecase example, assume that I would want to compute the cumulative maximum of a column and then see if the current value of the column is the highest one encountered so far. I could simply filter by pl.col('A') == pl.col('A_cum_max'), but this will not consider cases when the same value has been encountered before.

Proposal:
The most straightforward workaround that I have found is to just use a shifted cumulative maximum/whatever (i.e., cumulative maximum of the previous row in context), which may not be the easiest step computation-wise. Having some parameter such as right_closed=False (or maybe last_closed to not confuse when also using the reverse parameter) within the cum_* ops would save a lot of computation in this manner while (I believe and correct me if I am wrong please) not being so complicated to implement.
See the example for cum_max below taken from documentation along with my expected result:

df.with_columns(
    pl.col("a").cum_max().alias("cum_max"),
    pl.col("a").cum_max(right_closed=False).alias("cum_max_without_current_element"),
)
shape: (4, 3)
┌─────┬─────────┬────────────────────┐
│ a   ┆ cum_max ┆ cum_max_right_open │
│ --- ┆ ---     ┆ ---                │
│ i64 ┆ i64     ┆ i64                │
╞═════╪═════════╪════════════════════╡
│ 1   ┆ 1       ┆ null               │
│ 2   ┆ 2       ┆ 1                  │
│ 3   ┆ 3       ┆ 2                  │
│ 4   ┆ 4       ┆ 3                  │
└─────┴─────────┴────────────────────┘