KIP-1096 — Add ability to pass headers to ProducerPerformance

Discussion ProducerTesting

Adds `--headers` and `--headers-file` arguments to the `ProducerPerformance` CLI tool so benchmark runs can include custom record headers. Performance tests that must simulate production traffic (which includes headers carrying routing metadata, region tags, or consumer flags) have no way to include headers in `ProducerPerformance` runs, making benchmark results unrepresentative.

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