[OpenAI] [Scenario] Add conveniences for chaining Assistant operations via clientlets

[joseharriaga]

Consider adding clientlets that let you do things like this:

Thread thread = client.CreateThread();
ThreadMessage message = thread.AddMessage(new ThreadMessage(MessageRole.User, "How does AI work?"));
thread.Run(assistantId);
IReadOnlyList<ThreadMessage> newMessages = thread.GetMessages(after: message.Id);

Clientlets aren't directly constructible whereas subclients have .ctors