feat(rust,python,cli): add SQL engine support for IFNULL function

[alexander-beedie]

Support SQL "IFNULL" (equivalent to a two-argument "COALESCE").

Examples

import polars as pl

df = pl.DataFrame({
    "x": [1, None, 3],
    "y": [10, 20, 30],
})

pl.SQLContext(df=df).execute(
    """
    SELECT
      "x",
      IFNULL("x", "y") as "c1",
      IFNULL("x",-100) as "c2",
    FROM df
    """,
    eager=True,
)
# shape: (3, 3)
# ┌──────┬─────┬──────┐
# │ x    ┆ c1  ┆ c2   │
# │ ---  ┆ --- ┆ ---  │
# │ i64  ┆ i64 ┆ i64  │
# ╞══════╪═════╪══════╡
# │ 1    ┆ 1   ┆ 1    │
# │ null ┆ 20  ┆ -100 │
# │ 3    ┆ 3   ┆ 3    │
# └──────┴─────┴──────┘