KIP-1172 — Improve EndToEndLatency Tool with argument parser and message key/header support

Accepted Kafka 4.2 AdminTesting

Refactors the `EndToEndLatency` tool to use a proper argument parser (replacing fragile positional array indexing) and adds support for specifying message keys and headers in latency benchmark runs. The tool's positional argument parsing is brittle and makes it impossible to add optional parameters without breaking the argument order.

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