Implement return_field() for imported Rust UDFs

[andygrove]

Follow-up from review of #4459 (thread).

ImportedCScalarUdf::return_type already calls the kernel through the C ABI, decodes an FFI_ArrowSchema into a full Field, and then throws everything but the DataType away. The planner separately hardcodes the output field as nullable:

let return_field = Arc::new(Field::new(&call.name, kernel_return_type, true));

So a kernel that reports a non-nullable output, or attaches field metadata, has both discarded.

Implementing ScalarUDFImpl::return_field instead would carry the kernel's own Field through to the plan, and would let the planner stop fabricating one.

Worth checking before doing it:

• Spark UDF results are nullable in Spark's own schema, so promising DataFusion a non-nullable field needs to not trip a schema comparison at the Comet boundary or in the shuffle writer.
• The declared-vs-actual return type check in planner.rs deliberately erases nested nullability, because Spark carries containsNull in the type while the delivered array normalizes children to nullable. Whatever this does with top-level nullability should be consistent with that.
• A test for a kernel returning a non-nullable field, since nothing exercises that today.