Add a REVERSE function to PyDough

[vineetg3]

Goal: Add a REVERSE function to PyDough with the following functionality, which should be tested e2e (see test_pipeline.py for examples):

This should work with attributes of collections which are strings.

# Examples for Strings
REVERSE('hello')  # Output: 'olleh'
REVERSE('12345')  # Output: '54321'

[knassre-bodo]

For dialects that do not have a REVERSED function (such as sqlite), the following recursive CTE can be used to reverse a string t (and yes, the entire block below can be used as an expression):

WITH recursive reverse AS (
  SELECT -1 as i, '' as c, t as s
  UNION ALL
  SELECT i-1 as i, substr(s, i, 1) as c, s as s
  FROM reverse
  WHERE c!='' OR i = -1
)
SELECT group_concat(c, '') FROM reverse

For example, the following is a valid snippet of sqlite code:

WITH chartable(t) AS (values ('alphabet'), ('soup'), ('is'), ('delicious'), ('hello world'))
SELECT
    t,
    UPPER((
      WITH recursive reverse AS (
          SELECT -1 as i, '' as c, CONCAT('$', t, '*') as s
          UNION ALL
          SELECT i-1 as i, substr(s, i, 1) as c, s as s
          FROM reverse
          WHERE c!='' OR i = -1
      )
      SELECT group_concat(c, '') FROM reverse
    ))
from chartable

This snippet returns the following table:

alphabet|*TEBAHPLA$
soup|*PUOS$
is|*SI$
delicious|*SUOICILED$
hello world|*DLROW OLLEH$

Whatever string we wish to reverse is placed as the third argument in the values tuple inside reverse, whether this be a literal, column, or other expression. It should be fine if chartable also uses columns named i/c/s (just a bit confusing).

[knassre-bodo]

Use the implementation of #357 as a guideline.