feat(python): enhance expression-level filter syntax with support for multiple predicates and kwargs

[alexander-beedie]

Closes #12577 (following up #12603).

Adds the same support for multiple predicates and kwargs as recently added to frame-level filter and when/then.

Example

import polars as pl

df = pl.DataFrame({
    "key": ["a", "a", "a", "a", "b", "b", "b", "b", "b"],
    "n": [1, 2, 2, 3, 1, 3, 3, 2, 3],
})

df.group_by("key").agg(
    n_0 = pl.col("n").filter( n=1 ).sum(),
    n_1 = pl.col("n").filter( n=2 ).sum(),
    n_2 = pl.col("n").filter( n=3 ).sum(),
).sort(by="key")

# shape: (2, 4)
# ┌─────┬─────┬─────┬─────┐
# │ key ┆ n_0 ┆ n_1 ┆ n_2 │
# │ --- ┆ --- ┆ --- ┆ --- │
# │ str ┆ i64 ┆ i64 ┆ i64 │
# ╞═════╪═════╪═════╪═════╡
# │ a   ┆ 1   ┆ 4   ┆ 3   │
# │ b   ┆ 1   ┆ 2   ┆ 9   │
# └─────┴─────┴─────┴─────┘

[ritchie46]

Nice! I wait merging this when we merge the breaking changes ( soon ).

[alexander-beedie]

Nice! I wait merging this when we merge the breaking changes ( soon ).

No problem to wait, but this change isn't actually breaking ;)

[thomasfrederikhoeck]

This new style is very convinient but does it translate to the same filter? See #12932

[alexander-beedie]

This new style is very convinient but does it translate to the same filter? See #12932

Logically it did, but the optimiser didn't realise it could push the components down; should be fixed by #12976 👌

[alexander-beedie]

@ritchie46: rebased and added an "explain" check to confirm that we reduce all_horizontal nodes 👍

[ritchie46]

Great!