feat(function): add greatest function

[rluvaton]

Which issue does this PR close?

Closes #12472

Rationale for this change

Support more operators

What changes are included in this PR?

Implement the Spark implementation for greatest:
https://spark.apache.org/docs/latest/api/python/reference/pyspark.sql/api/pyspark.sql.functions.greatest.html

Are these changes tested?

I was not able to find where you put the functions test, the coalesce for example has only one test for the return type, I must missing something

datafusion/datafusion/functions/src/core/coalesce.rs

Lines 142 to 157 in 7bd7747

	mod test {
	use arrow::datatypes::DataType;
	use datafusion_expr::ScalarUDFImpl;
	use crate::core;
	#[test]
	fn test_coalesce_return_types() {
	let coalesce = core::coalesce::CoalesceFunc::new();
	let return_type = coalesce
	.return_type(&[DataType::Date32, DataType::Date32])
	.unwrap();
	assert_eq!(return_type, DataType::Date32);
	}
	}

Are there any user-facing changes?

Yes, I need to add documentation, but first check if this feature is desired

[Weijun-H]

FYI #12357 (comment)

[comphead]

Related to #6531

[rluvaton]

I was not able to find where you put the functions test, the coalesce for example has only one test for the return type, I must missing something

Hey @alamb, we talked about the test location at the beginning of the CMU talk you gave, where the tests are located?

[alamb]

I think the tests I was talking about are described here:

https://github.com/apache/datafusion/tree/4a3c09a9316fb8940aeb1c5b9b48bc3b7259d5d4/datafusion/sqllogictest#readme

If you want to test the type of a function you can do it like

select arrow_typeif(my_func(foo)))

[rluvaton]

I see, thank you, how is your experience with debugging this kind of test?

[alamb]

It is great!