Support unix_date and date_from_unix_date natively

[Sigma-Ma]

Is your feature request related to a problem? Please describe.

Auron does not currently support native execution of Spark's unix_date and date_from_unix_date expressions. These expressions fall back to Spark evaluation.

Describe the solution you'd like

Add native support for both functions:

• unix_date(date) returns an integer day count relative to 1970-01-01.
• date_from_unix_date(days) converts that day count back to a date.

Preserve Spark's return types and null behavior. Add tests for zero, positive and negative day counts, integer boundaries, and round-trip conversions. Use column inputs in Spark regression tests and verify that both expressions execute natively.

Describe alternatives you've considered

Keep the existing Spark fallback.

Additional context

Both functions were introduced in Spark 3.1.0.

SELECT unix_date(DATE '1970-01-01'); -- 0

SELECT unix_date(DATE '1969-12-31'); -- -1

SELECT date_from_unix_date(0); -- 1970-01-01

SELECT date_from_unix_date(-1); -- 1969-12-31