feat(server): Add k6 Load Testing Script

[sachaarbonel]

Summary

• Added bench.js k6 script to examples/server/ for load testing the Whisper HTTP server with concurrent multipart requests.
• Updated examples/server/README.md with a new section detailing how to run the benchmark, including example commands and environment variable explanations.
• Ensured the script is configurable and works with standard k6 installations.

Motivation

This addition enables easy benchmarking and stress testing of the Whisper server’s /inference endpoint, helping users and developers validate performance under concurrent load.

[sachaarbonel]

@danbev done