Add CLI options for backend args (like headers and verify)

[kdelee]

This PR adds the ability to configure custom request headers and control SSL certificate verification when running benchmarks.

• The OpenAIHTTPBackend now supports passing custom headers and a verify flag to disable SSL verification.
• Headers are now merged with the following precedence: CLI arguments (--backend-args), scenario file arguments, environment variables, and then default values.
• Headers can be removed by setting their value to null in the --backend-args JSON string.
• The --backend-args help text has been updated with an example of how to use these new features.
• New documentation has been added for the CLI, configuration options, and supported data formats.
• Unit tests have been added to verify the new header and SSL verification logic, as well as the CLI argument parsing.

This provides a way to benchmark targets that require custom authentication, other headers, or use self-signed SSL certificates.

[sjmonson]

Hey, thanks for the contribution. For backend specific arguments, please nest them under --backend_args and handle any parsing within the backend's __init__. Please also replace all occurrences of ssl with cert or some other protocol neutral term.

[kdelee]

Ok thanks for feedback, will look into those changes

[kdelee]

@sjmonson pushed some updates, looks like workflow needs approval

[sjmonson]

Overall looks pretty good, just a few design nits. To fix markdown linting run python -m mdformat <files>

[sjmonson]

LGTM but commits need signoff