Pseudooo/htrs

AI Summary: Implement a default User-Agent header of `htrs/<version>` for an HTTP CLI, allowing users to override it with custom headers.

AI Summary: Implement support for HTTP headers in an existing HTTP CLI tool. Users should be able to specify headers using `-h` or `--header` flags with key-value pairs (e.g., `-h Header1=Foo --header Header2=Bar`).

AI Summary: Modify the HTTP CLI to correctly handle the configuration file path. The current implementation only searches for `./htrs_config.json` in the executable's directory. The fix should use `std::env::current_exe()` to determine the executable's path, allowing the config file to be found regardless of the working directory. Error handling should be implemented for cases where the config file is not found.