chore: add count distinct group benchmarks

[coderfender]

Which issue does this PR close?

Add benchmarks for group accumulators to test : #21561
The implementation forks out based on is_groups_accumulator_supported function call. Once this is merged , we should be able to evaluate group accumulators on count distinct expr

• Closes #.

Rationale for this change

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

[coderfender]

@Dandandan , I plan to add benches to help better evaluate group accumulators along with direct clickbench / TPCH queries for implementing group accumulators . Please take a look whenever you get a chance

[martin-g]

Suggested change

	if let Some(val) = arr.value(idx).into() {
	let single_val =
	Arc::new(Int64Array::from(vec![Some(val)])) as ArrayRef;
	accumulators[*group_idx]
	.update_batch(std::slice::from_ref(&single_val))
	.unwrap();
	}
	let single_val = values.slice(idx, 1);
	accumulators[*group_idx]
	.update_batch(std::slice::from_ref(&single_val))
	.unwrap();

slightly simpler and would avoid the allocation of the single valued array for each row

[martin-g]

Another way would be to collect per-group indices first and then build the array:

let mut group_rows: Vec<Vec<i64>> = vec![Vec::new(); num_groups];
for (idx, &group_idx) in group_indices.iter().enumerate() {
    if arr.is_valid(idx) {
        group_rows[group_idx].push(arr.value(idx));
    }
}
for (group_idx, rows) in group_rows.iter().enumerate() {
    if !rows.is_empty() {
        let batch = Arc::new(Int64Array::from(rows.clone())) as ArrayRef;
        accumulators[group_idx].update_batch(std::slice::from_ref(&batch)).unwrap();
    }
}

[martin-g]

Why do you need an Option here ?
arr.value(idx) returns i64 and calling .into() always return Some.
If you want to filter out the nulls then you need to use arr.is_null(idx)

[martin-g]

nit: Introduce an enum instead of using strings:

enum GroupDist {
    Uniform, 
    Skewed 
}