configurable latency percentiles

[GabrielTecuceanu]

Latency percentiles are currently hardcoded. Users running SLO-sensitive benchmarks may need p99.9 or custom percentiles.

What to add:
cli:

httpress -n 10000 https://example.com --percentiles 50,90,99,99.9

library:

Benchmark::builder()
    .url("https://example.com")
    .percentiles(vec![50.0, 90.0, 99.0, 99.9])
    .build()?
    .run()
    .await?;

Relevant files:

• src/cli.rs - add flag
• src/config.rs - add --percentiles field with default of [50.0, 90.0, 95.0, 99.0]
• src/benchmark.rs - add .percentiles() builder method
• src/metrics.rs- make percentile computation dynamic
• src/main.rs - update display

notes:

• make sure to also update the readme cli useage / options section to include the new flag
• also add a documentation comment to .percentiles() builder method with a description and a minimal example (you can look at the other functions for reference).