Add field trait method to WindowUDFImpl

[jcsherin]

Is your feature request related to a problem or challenge?

Instead of return_type + nullable. I think field is a better choice. Given windowudf is pretty new so not widely used yet (?). It would be nice to have field(&self, args: FieldArgs) -> Result<Field>

FieldArgs {
 arg_type: &[DataType]
 schema: Schema // for nullable
}

It is too late for AggregateUDF and ScalarUDF :(

Originally posted by @jayzhan211 in #12030 (comment)

This is a proposal to add a field trait method for implementing user-defined window functions. And also remove (or deprecate) the trait methods return_type and nullable from WindowUDFImpl.

Both the return data type and nullability of the result from evaluating the user-defined window function will be specified by the field implementation.

Describe the solution you'd like

The current implementation for a user-defined window function (without field trait method) looks like this:

impl WindowUDFImpl for RowNumber {
    fn return_type(&self, _arg_types: &[DataType]) -> Result<DataType> {
        Ok(DataType::UInt64)
    }

    fn nullable(&self) -> bool {
        true
    }
}

The implementation for a user-defined window function after this change:

impl WindowUDFImpl for RowNumber {
    fn field(&self, field_args: WindowUDFFieldArgs) -> Result<Field> {
        Ok(Field::new(
             field_args.name(), /* window function display name */
             DataType::UInt64,  /* result data type */
             false              /* row number is not nullable */
       ))
    }
}

---

1. Add the field trait method to WindowUDFImpl:

fn field(&self, field_args: WindowUDFFieldArgs) -> Result<Field>

2. Add WindowUDFFieldArgs:

/// Contains metadata necessary for defining the field which represents
/// the final result of evaluating a user-defined window function.
pub struct WindowUDFFieldArgs<'a> {
    /// The data types of input expressions to the user-defined window
    /// function.
    input_types: &'a [DataType],
    /// The display name of the user-defined window function.
    function_name: &'a str,
}

3. Remove/Deprecate return_type trait method from WindowUDFImpl:
   

   datafusion/datafusion/expr/src/udwf.rs

   Lines 282 to 284 in 502ce4b

   	/// What [`DataType`] will be returned by this function, given the types of
   	/// the arguments
   	fn return_type(&self, arg_types: &[DataType]) -> Result<DataType>;

4. Remove nullable trait method from WindowUDFImpl:
   

   datafusion/datafusion/expr/src/udwf.rs

   Lines 347 to 354 in 502ce4b

   	/// Allows customizing nullable of column for this window UDF.
   	///
   	/// By default, the final result of evaluating the window UDF is
   	/// allowed to have null values. But if that is not the case then
   	/// it can be customized in the window UDF implementation.
   	fn nullable(&self) -> bool {
   	true
   	}

Describe alternatives you've considered

Make no changes to WindowUDFImpl.

Additional context

Part of #8709.
Follow up to #12030.