Reduce repetition in `try_process_group_by_unnest` and `try_process_unnest`

[alamb]

This function seems to have some code repetition with function try_process_unnest.

I wonder if there's a better way to handle this, such as planning unnest before aggregation, and then reusing the current group-by planning logic. This seems more intuitive to me. But I'm not sure about it.

Originally posted by @jonahgao in #11469 (comment)

The idea is to remove the repetition, and possibly also this comment as well: https://github.com/apache/datafusion/pull/11469/files#r1678995060

If unnest has already been processed by try_process_aggregate_unnest, does the following logic for handling unnest become redundant?

[JasonLi-cn]

take

[JasonLi-cn]

After testing, it is found that the following logic is necessary even if the unnest has been processed by try_process_aggregate_unnest. Because try_process_aggregate_unnest deals with unnest in the Aggregate's input, and the following logic deals with unnest in select_exprs, which is downstream of the Aggregate. So we still need this piece of logic:

datafusion/datafusion/sql/src/select.rs

Lines 311 to 344 in 2f5e73c

	let mut unnest_columns = vec![];
	// from which column used for projection, before the unnest happen
	// including non unnest column and unnest column
	let mut inner_projection_exprs = vec![];
	// expr returned here maybe different from the originals in inner_projection_exprs
	// for example:
	// - unnest(struct_col) will be transformed into unnest(struct_col).field1, unnest(struct_col).field2
	// - unnest(array_col) will be transformed into unnest(array_col).element
	// - unnest(array_col) + 1 will be transformed into unnest(array_col).element +1
	let outer_projection_exprs: Vec<Expr> = intermediate_select_exprs
	.iter()
	.map(|expr| {
	transform_bottom_unnest(
	&intermediate_plan,
	&mut unnest_columns,
	&mut inner_projection_exprs,
	expr,
	)
	})
	.collect::<Result<Vec<_>>>()?
	.into_iter()
	.flatten()
	.collect();
	// No more unnest is possible
	if unnest_columns.is_empty() {
	// The original expr does not contain any unnest
	if i == 0 {
	return LogicalPlanBuilder::from(intermediate_plan)
	.project(inner_projection_exprs)?
	.build();
	}
	break;