Implement native function of datediff

[Sigma-Ma]

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

Spark datediff(endDate, startDate) is not supported in Auron's native execution path. NativeConverters does not handle Spark's DateDiff expression, so queries using it remain on the fallback path.

Describe the solution you'd like

Add Spark-compatible native support for datediff(endDate, startDate) through the extension-function path:

• convert Spark DateDiff in NativeConverters
• register and implement a native Spark_DateDiff function
• return the signed number of days from startDate to endDate
• preserve null propagation and Spark input coercion behavior
• add coverage in AuronFunctionSuite and Rust unit tests

Expected examples:

SELECT datediff(DATE '2009-07-31', DATE '2009-07-30'); -- 1
SELECT datediff(DATE '2009-07-30', DATE '2009-07-31'); -- -1
SELECT datediff(DATE '2024-03-01', DATE '2024-02-28'); -- 2
SELECT datediff(CAST(NULL AS DATE), DATE '2024-01-01'); -- NULL

Tests should cover positive, negative, and zero differences, leap-year and year-boundary dates, and null inputs.

Describe alternatives you've considered

Keep the current Spark fallback. This preserves correctness, but datediff cannot benefit from native execution.

Additional context

Related native date-function implementations:

• [AURON #2128] Implement native function of dayofweek #2129: dayofweek
• [AURON #2485] Support last_day function natively #2486: last_day
• [AURON #2488] Support dayofyear function natively #2489: dayofyear

[Sigma-Ma]

I'd like to work on this issue.

[Sigma-Ma]

Implemented in #2502. The PR adds native DateDiff conversion, the Spark_DateDiff implementation, and Rust and Spark regression coverage. It is ready for review.