Add OpenAI assistant role to chat completions API

[lzell]

Sample request:

let openAIService = AIProxy.openAIService(
    partialKey: "partial-key-from-your-developer-dashboard",
    serviceURL: "service-url-from-your-developer-dashboard"
)
do {
    let response = try await openAIService.chatCompletionRequest(body: .init(
        model: "gpt-4o-mini",
        messages: [.assistant(content: .text("hello world"))]
    ))
    print(response.choices.first?.message.content ?? "")
}  catch AIProxyError.unsuccessfulRequest(let statusCode, let responseBody) {
    print("Received non-200 status code: \(statusCode) with response body: \(responseBody)")
} catch {
    print(error.localizedDescription)
}