✨(gentest): allow custom headers in gentest config file and add an example config file

[raxhvl]

1. Instead of using specific fields like client_id and secret, I propose we change the config file structure to include a single key called rpc_headers. This allows users to define a flexible number of headers as needed.

{
    "remote_nodes": [
        {
            "name": "mainnet_archive",
            "node_url": "https://abc.com",
            "rpc_headers": {
                "Authorization": "Bearer your_token_here",
                "Another-Header": "value"
            }
        }
    ]
}

2. Create a config.json.example file that serves as a template for new users. This will help them quickly understand how to structure their configuration.