feat(benchmarks): add --config to tpch datafusion + contributor guide

[andygrove]

Which issue does this PR close?

Closes #.

Rationale for this change

Two related gaps for contributors who want to benchmark Ballista:

1. The tpch benchmark datafusion subcommand has no way to override DataFusion session config keys. The tpch benchmark ballista subcommand has had a -c key=value flag for this since chore(deps): update to datafusion v.53 #1486 era, but the in-process DataFusion path requires editing source to change a single config. That makes apples-to-apples comparisons between the two modes hard.
2. The contributor development guide currently has a one-paragraph Benchmarking section that links out to benchmarks/README.md. The README covers cluster setup but not the day-to-day contributor workflow: which subcommand to use, how to override configs, how to read the metrics, and how to capture a flame graph when something is unexpectedly slow.

What changes are included in this PR?

• Code: benchmarks/src/bin/tpch.rs adds a repeatable -c/--config key=value flag to the datafusion subcommand, mirroring the one already on ballista. Overrides are applied to the SessionConfig before constructing the SessionContext. Invalid keys log a warning and are skipped, matching the existing behavior on the ballista path.
• Docs: New page docs/source/contributors-guide/benchmarking.md covering:
  • Generating TPC-H input with tpchgen-rs
  • Running the tpch binary in benchmark datafusion (in-process) and benchmark ballista (against a cluster) modes, with notes on picking a representative single query while iterating
  • Setting session configs via -c key=value, with a table of commonly tuned datafusion.* and ballista.* keys
  • Reading metrics and a pointer to the existing Metrics user guide
  • Profiling with cargo flamegraph and samply
• The Benchmarking section in contributors-guide/development.md now points at the new page and keeps the link to benchmarks/README.md
• The new page is registered in the index.rst toctree under Contributors Guide

Are there any user-facing changes?

tpch benchmark datafusion accepts a new -c/--config flag. No existing flags or defaults change, so prior invocations behave the same.