Add configurable UTF8 payload profiles to string_agg benchmarks and parameterize benchmark matrix

[kosiew]

Which issue does this PR close?

• Part of Consider deferred copying in string_agg GroupsAccumulator #21156

---

Rationale for this change

The current string_agg benchmarks use very small (≈3-byte) string payloads (e.g., hi0..hi3). This makes it difficult to observe the real cost of string aggregation in scenarios where payload sizes are larger, particularly the cost of copying and memory pressure during grouped aggregation.

This PR introduces configurable UTF-8 payload sizes so benchmarks can better reflect realistic workloads and expose CPU and memory behavior differences across payload sizes.

---

What changes are included in this PR?

• Introduced a new Utf8PayloadProfile enum with three profiles:

  • Small (≈3 bytes, existing baseline)
  • Medium (≈64 bytes)
  • Large (≈1024 bytes)

• Added create_table_provider_with_payload to allow generating tables with configurable string payload sizes.

• Refactored record batch generation to use precomputed payload arrays instead of formatting strings per row.

• Added helper:

  • payload_string for generating fixed-size string payloads
  • Utf8PayloadProfile::payloads() for producing 4-value low-cardinality payload sets

• Updated benchmark setup:

  • Introduced create_context_with_payload

  • Parameterized string_agg queries across:

    • group cardinality (few, mid, many)
    • payload size (small_3b, medium_64b, large_1024b)

• Replaced individual benchmark functions with a criterion benchmark group using BenchmarkId to produce a matrix of results.

---

Are these changes tested?

No new unit tests were added.

Reason:

• This change only affects benchmark utilities and benchmark definitions.
• Existing functionality remains unchanged for default (Small) payload profile.
• Benchmarks themselves act as validation for correctness and performance behavior.

---

Are there any user-facing changes?

No.

This PR only impacts internal benchmarking infrastructure and does not modify public APIs or query behavior.

---

LLM-generated code disclosure

This PR includes LLM-generated code and comments. All LLM-generated content has been manually reviewed and tested.

---

Additional Notes

• The benchmark matrix now isolates the impact of payload size while preserving low cardinality (4 distinct values), ensuring that observed differences are primarily due to string size rather than grouping distribution.
• Medium payloads aim to expose copy costs without excessive allocator overhead, while large payloads stress both CPU and memory behavior.

---