feat: add native GroupsAccumulator for any_value

[Kevin-Li-2025]

Which issue does this PR close?

Follow-up to #23043. This draft is stacked on that PR and should be reviewed after it merges.

Rationale for this change

Grouped any_value currently falls back to GroupsAccumulatorAdapter, which creates one boxed Accumulator per group, collects row indices, materializes per-group slices, and performs dynamic dispatch for every group. That overhead dominates high-cardinality GROUP BY workloads even though any_value only needs to retain one non-null value per group.

This PR adds a native GroupsAccumulator that:

• stores one ScalarValue and one is_set bit per group;
• scans each input batch once and stops updating a group after its first valid value;
• preserves the existing two-column partial-state contract;
• supports filters, state merging, EmitTo::First, and convert_to_state; and
• works for every Arrow type accepted by any_value.

What changes are included?

• Native grouped accumulator implementation and four focused unit tests.
• Criterion benchmark comparing the native path with GroupsAccumulatorAdapter for Int64 and Utf8 at 8,192 rows / 4,096 groups.

Local Apple Silicon benchmark medians:

Type	Native	Adapter	Improvement
Int64	0.245 ms	4.92 ms	~20x
Utf8	0.515 ms	12-13 ms	>10x

The Utf8 adapter result is allocator-sensitive, so the claim is intentionally conservative.

Validation

• cargo test -p datafusion-functions-aggregate --lib (146 passed)
• cargo clippy -p datafusion-functions-aggregate --all-targets -- -D warnings
• cargo fmt --all -- --check
• cargo bench -p datafusion-functions-aggregate --bench any_value -- --noplot
• git diff --check