tech debt: benchmark storing records in memory

[fraidev]

The Fluvio benchmark tool currently stores all generated records in memory during benchmarking. When running benchmarks with a high num-records value, this approach can quickly exhaust available memory, causing system instability or crashes.

Suggested Improvements:

• Write records to a temporary file instead of keeping them entirely in memory.
• Alternatively, generate records on-the-fly during benchmarking, avoiding large memory allocations.