Aggregate function via FFI calls `return_type()` instead of `return_field()`

[paleolimbot]

Describe the bug

I'm not sure if this is intentional, but this code:

datafusion/datafusion/ffi/src/udaf/mod.rs

Lines 163 to 177 in d83a290

	unsafe extern "C" fn return_type_fn_wrapper(
	udaf: &FFI_AggregateUDF,
	arg_types: RVec<WrappedSchema>,
	) -> RResult<WrappedSchema, RString> {
	let udaf = udaf.inner();
	let arg_types = rresult_return!(rvec_wrapped_to_vec_datatype(&arg_types));
	let return_type = udaf
	.return_type(&arg_types)
	.and_then(|v| FFI_ArrowSchema::try_from(v).map_err(DataFusionError::from))
	.map(WrappedSchema);
	rresult!(return_type)
	}

...triggered a failure in one of my tests because it calls return_type() instead of return_field(). I had been making the assumption that if return_field() was implemented then return_type() shouldn't be called, so I had been returning an error from that function.

I believe that the FFI schemas should be able to carry field metadata...should return_field() be called here?

To Reproduce

Implement an aggregate function that returns an extension type. I believe that when this happens, the metadata on output will be lost or the source return_type() will be called instead of the return_field() implementation.

Expected behavior

I would have expected the field information to propagate to the output.

Additional context

cc @timsaucer ...help!

[timsaucer]

This looks like an oversight when we updated all of the return field code.