Functionality to add custom headers

[wtlow003]

Is there any existing functionality to add custom headers for custom api that uses openai or customopenai as a llm_provider?

I am expecting that equivalent CURL command would be similar to:

curl -X POST {api_base} \
    -H Authorization Bearer {custom_token} \
    -H {custom_header} {custom_value}

As opposed to using the proxy_server_config.yaml, where I add headers under litellm-params, which the headers will appear under the {extra_headers}

[ishaan-jaff]

Done here #2164