feat: Add struct.with_fields

[ritchie46]

Akin to DataFrame.with_columns.

Will follow up with a select_fields as well.

>>> df = pl.DataFrame(
...     {
...         "coords": [{"x": 1, "y": 4}, {"x": 4, "y": 9}, {"x": 9, "y": 16}],
...         "multiply": [10, 2, 3],
...     }
... )
>>> df
shape: (3, 2)
┌───────────┬──────────┐
│ coords    ┆ multiply │
│ ---       ┆ ---      │
│ struct[2] ┆ i64      │
╞═══════════╪══════════╡
│ {1,4}     ┆ 10       │
│ {4,9}     ┆ 2        │
│ {9,16}    ┆ 3        │
└───────────┴──────────┘
>>> df = df.with_columns(
...     pl.col("coords").struct.with_fields(
...         pl.col("coords").struct.field("x").sqrt(),
...         y_mul=pl.col("coords").struct.field("y") * pl.col("multiply"),
...     ),
... )
>>> df
shape: (3, 2)
┌─────────────┬──────────┐
│ coords      ┆ multiply │
│ ---         ┆ ---      │
│ struct[3]   ┆ i64      │
╞═════════════╪══════════╡
│ {1.0,4,40}  ┆ 10       │
│ {2.0,9,18}  ┆ 2        │
│ {3.0,16,48} ┆ 3        │
└─────────────┴──────────┘
>>> df.unnest("coords")
shape: (3, 4)
┌─────┬─────┬───────┬──────────┐
│ x   ┆ y   ┆ y_mul ┆ multiply │
│ --- ┆ --- ┆ ---   ┆ ---      │
│ f64 ┆ i64 ┆ i64   ┆ i64      │
╞═════╪═════╪═══════╪══════════╡
│ 1.0 ┆ 4   ┆ 40    ┆ 10       │
│ 2.0 ┆ 9   ┆ 18    ┆ 2        │
│ 3.0 ┆ 16  ┆ 48    ┆ 3        │
└─────┴─────┴───────┴──────────┘

[alexander-beedie]

Love it! Also, a select_fields addition would probably close this one: #3859 🤔

[ritchie46]

Ah, that seems like a special case of expansion. I think that should be fairly easy. Will take a look. And ping me if I forget.