perf: Add support for GroupsAccumulator to string_agg (#21154)

## Which issue does this PR close?

- Closes #17789.

## Rationale for this change

`string_agg` previously didn't support the `GroupsAccumulator` API;
adding support for it can significantly improve performance,
particularly when there are many groups.

Benchmarks (M4 Max):

 - string_agg_query_group_by_few_groups (~10): 645 µs → 564 µs, -11%
- string_agg_query_group_by_mid_groups (~1,000): 2,692 µs → 871 µs, -68%
- string_agg_query_group_by_many_groups (~65,000): 16,606 µs → 1,147 µs,
-93%

## What changes are included in this PR?

* Add end-to-end benchmark for `string_agg`
* Implement `GroupsAccumulator` API for `string_agg`
* Add unit tests
* Minor code cleanup for existing `string_agg` code paths

## Are these changes tested?

Yes.

## Are there any user-facing changes?

No, other than a change to an error message string.

---------

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

Author: neilconway

datafusion/core/benches/aggregate_query_sql.rs

@@ -295,6 +295,39 @@ fn criterion_benchmark(c: &mut Criterion) {
             )
         })
     });
+
+    c.bench_function("string_agg_query_group_by_few_groups", |b| {
+        b.iter(|| {
+            query(
+                ctx.clone(),
+                &rt,
+                "SELECT u64_narrow, string_agg(utf8, ',') \
+                 FROM t GROUP BY u64_narrow",
+            )
+        })
+    });
+
+    c.bench_function("string_agg_query_group_by_mid_groups", |b| {
+        b.iter(|| {
+            query(
+                ctx.clone(),
+                &rt,
+                "SELECT u64_mid, string_agg(utf8, ',') \
+                 FROM t GROUP BY u64_mid",
+            )
+        })
+    });
+
+    c.bench_function("string_agg_query_group_by_many_groups", |b| {
+        b.iter(|| {
+            query(
+                ctx.clone(),
+                &rt,
+                "SELECT u64_wide, string_agg(utf8, ',') \
+                 FROM t GROUP BY u64_wide",
+            )
+        })
+    });
 }
 
 criterion_group!(benches, criterion_benchmark);