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

[alexander-beedie]

Add CONCAT_WS¹ support to the SQL engine (aka: "concat with separator").

Interleaving a separator is common when concatenating; this function allows for that with a more lightweight syntax than applying the separators explicitly.

Example

import polars as pl

df = pl.DataFrame({
    "x": ["a", "b", "c", "d"],
    "y": ["e", "f", "g", "h"],
    "z": ["i", "j", "k", "l"],
})

pl.SQLContext(data=df).execute(
    """
    SELECT
      ("x" || ':' || "y" || ':' || "z") AS c1,   -- concat operator
      CONCAT("x", ':', "y", ':', "z")   AS c2,   -- basic concat function
      CONCAT_WS(':', "x", "y", "z")     AS c3,   -- concat 'with separator' variant
    FROM data
    """,
    eager=True,
)

# shape: (4, 3)
# ┌───────┬───────┬───────┐
# │ c1    ┆ c2    ┆ c3    │
# │ ---   ┆ ---   ┆ ---   │
# │ str   ┆ str   ┆ str   │
# ╞═══════╪═══════╪═══════╡
# │ a:e:i ┆ a:e:i ┆ a:e:i │
# │ b:f:j ┆ b:f:j ┆ b:f:j │
# │ c:g:k ┆ c:g:k ┆ c:g:k │
# │ d:h:l ┆ d:h:l ┆ d:h:l │
# └───────┴───────┴───────┘

Footnotes

1. PostgreSQL docs: concat_ws ↩