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

[alexander-beedie]

Adds sign support to the SQL engine (returns -1, 0, +1 depending on the sign of the argument).

Example

import polars as pl

df = pl.DataFrame({"n": [-123, None, 0, 404]})

with pl.SQLContext(data=df) as ctx:
    ctx.execute("""SELECT n, SIGN(n) AS "pos_neg" FROM data""").collect()
    
# shape: (4, 2)
# ┌──────┬─────────┐
# │ n    ┆ pos_neg │
# │ ---  ┆ ---     │
# │ i64  ┆ i64     │
# ╞══════╪═════════╡
# │ -123 ┆ -1      │
# │ null ┆ null    │
# │ 0    ┆ 0       │
# │ 404  ┆ 1       │
# └──────┴─────────┘

---

Misc: sorted the '# extra operations' section in the main polars Cargo.toml for clarity; likely worth doing systematically across the larger Cargo.toml blocks in all crates (currently just doing it opportunistically as I encounter them 🤔)