Respect custom http headers for outgoing client requests

[simonjanssen]

I am operating in a env where I need to set custom headers for my LLM providers (e.g. user-agent, etc.). Specifically, I need to set a custom Accept header for streaming updates via OpenAI clients.

Setting default headers for my reqwest client had no effect as the ProviderClient treats http client and http headers separately for http request building.

In the ClientBuilder these headers default to an empty map. As far as I can tell there is currently no public setter method to set anything but the default headers.

What I've added/changed:

• added a demo http_header setter method for ClientBuilder to add additional, custom http headers
• GenericEventSource::new() reflects custom Accept headers set this way instead of overwriting

Tested the proposed changes with this setup which worked just fine:

let client = openai::CompletionsClient::<reqwest::Client>::builder()
    .base_url(my_base_url)
    .http_headers(my_custom_headers) // added
    .http_client(my_reqwest_client)
    .api_key(my_api_key)
    .build()?;

[joshua-mo-143]

Needs cargo fmt then lgtm

[simonjanssen]

Needs cargo fmt then lgtm

done

[joshua-mo-143]

lgtm, thanks