feat: Add SQL syntax support for CROSS JOIN UNNEST(col)

[alexander-beedie]

Closes #21180.

This syntax represents a simple lateral unnest (explode) against the left join table and the referenced column, so this PR adds a small/dedicated path for handling it. Added suitable test coverage.

Example

import polars as pl

df = pl.DataFrame({
    "id": ["xyz", "abc"],
    "items": [[100, 200], [300, 400, 500]],
})

df.sql("""
  SELECT id, item
  FROM self CROSS JOIN UNNEST(items) AS item
  ORDER BY id DESC, item ASC
""")
# shape: (5, 2)
# ┌─────┬──────┐
# │ id  ┆ item │
# │ --- ┆ ---  │
# │ str ┆ i64  │
# ╞═════╪══════╡
# │ xyz ┆ 100  │
# │ xyz ┆ 200  │
# │ abc ┆ 300  │
# │ abc ┆ 400  │
# │ abc ┆ 500  │
# └─────┴──────┘

---

Also: automatic pytest.skip if running tests locally that use assert_sql_matches with duckdb, if duckdb is not installed (note: will always be available on CI).

[codecov]

Codecov Report

❌ Patch coverage is 88.33333% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.53%. Comparing base (cba9324) to head (34cb152).

Files with missing lines	Patch %	Lines
crates/polars-sql/src/context.rs	88.33%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25623      +/-   ##
==========================================
+ Coverage   79.49%   79.53%   +0.03%     
==========================================
  Files        1742     1742              
  Lines      240147   240207      +60     
  Branches     3038     3038              
==========================================
+ Hits       190909   191049     +140     
+ Misses      48455    48375      -80     
  Partials      783      783              

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