feat(rust,python,cli): add SQL engine support for RIGHT and REVERSE string functions

[alexander-beedie]

Updates

• We already support LEFT and SUBSTR; this PR adds the missing support for RIGHT.
• Also adds support for the REVERSE string function.
• As the primary SQL tests file recently exceeded 1000 lines, I channelled my inner @stinodego and overhauled the directory structure 😉 Instead of one large file it has been broken out into a set of fifteen. This will help identify weaker areas of coverage and better-organise future tests.

Example

import polars as pl

df = pl.DataFrame({"txt": ["abcdefg"]})

with pl.SQLContext({"data": df}) as sql:
    sql.execute(
        """
        SELECT 
          txt,
          LEFT(txt,3) AS "l", 
          RIGHT(txt,3) AS "r",
          REVERSE(txt) AS "rev"
        FROM data
        """,
        eager=True
    )
    # shape: (1, 4)
    # ┌─────────┬─────┬─────┬─────────┐
    # │ txt     ┆ l   ┆ r   ┆ rev     │
    # │ ---     ┆ --- ┆ --- ┆ ---     │
    # │ str     ┆ str ┆ str ┆ str     │
    # ╞═════════╪═════╪═════╪═════════╡
    # │ abcdefg ┆ abc ┆ efg ┆ gfedcba │
    # └─────────┴─────┴─────┴─────────┘