bench: add key-only payload sort benchmarks

[gratus00]

Which issue does this PR close?

Partially addresses #21543.

Rationale for this change

#21543 points out that the existing sort benchmark does not cover an important ExternalSorter shape: sorting by a cheap key while carrying non-key payload columns through the sort

The existing tuple/string/dictionary cases sort by every column via make_sort_exprs(schema). That is useful coverage, but it does not model queries like ORDER BY key over rows that also contain UTF-8 or dictionary payload columns. This PR adds benchmark coverage for that case, following the discussion in #21543 and the benchmark/use-case direction from #21688

I used Codex to help draft parts of this benchmark change and the PR description. I reviewed and adjusted the resulting code locally before opening the PR

What changes are included in this PR?

• Change the sort benchmark batch size from 1024 to 8192, matching DataFusion's default target batch size more closely
• Add key-only sort benchmark cases:
  • i64 key utf8 payload
  • i64 key dictionary payload
• Add selected-column sort helpers for those new benchmark cases, so they sort only by key while carrying payload columns
• Keep the existing benchmark cases sorting all columns as before

The new cases are run across the existing four benchmark shapes:

• merge sorted
• sort merge
• sort
• sort partitioned

Are these changes tested?

Focused checks run locally:

cargo fmt --all --check
cargo check -p datafusion --bench sort
cargo clippy -p datafusion --bench sort --all-features -- -D warnings

Are there any user-facing changes?

No. This is a benchmark-only change

[gratus00]

@mbutrovich would really appreciate your review on this, I think there might be room for improvement here