Adding additional parameters in API.

[jamesrochabrun]

Consumers now can pass additional parameters in to the request. e.g Open Router:

let service = OpenAIServiceFactory.service(
              apikey:  "${OPENROUTER_API_KEY}",  
             baseURL: "https://openrouter.ai",
             proxyPath: "api"
             headers: ["HTTP-Referer":  "${YOUR_SITE_URL}",  "X-Title":  "${YOUR_SITE_NAME}")