Add Support for Custom Headers

[njegosrailic]

What

The current implementation lacks the ability to add custom headers to HTTP requests. Adding this capability can be useful for various purposes, such as passing custom client information or other metadata.

Description

This feature would enhance the flexibility and utility of the application by allowing users to specify additional headers as needed.

We propose introducing a new command-line flag, --custom-headers, which accepts a comma-separated list of headers in key=value format. The headers will be parsed and added to the HTTP request only if they do not already exist in the request, ensuring that the original headers are not overridden.