Add serviceURL to AIProxy integration

[lzell]

• Starting next week, all AIProxy integrations will require a service URL. Developers each get their own service URL to proxy requests through. The service URL is displayed in the AIProxy dashboard when you configure your project.
• The AIProxy integration code changes from this:

let service = OpenAIServiceFactory.service(
    aiproxyPartialKey: "your_partial_key_goes_here"
)

to this:

let service = OpenAIServiceFactory.service(
    aiproxyPartialKey: "your_partial_key_goes_here",
    aiproxyServiceURL: "your_service_url_goes_here"
)