Add support for mocking via the OpenAIModelFactory

[joseharriaga]

As of version 2.0.0-beta.7, there is no simple way for users to mock service outputs for testing. Consequently, they currently rely on workarounds such as using ModelReaderWriter.Read<T> to create a mock from JSON as illustrated below:

ChatCompletion completion = ModelReaderWriter.Read<ChatCompletion>(BinaryData.FromObjectAsJson(new {
  id = "1234",
  choices = new object[]
  {
      new {
          finish_reason = "stop",
          index = 0,
          message = new {
              content = "It's a nice day today!",
              role = "assistant"
          }
      }
  },
  created = DateTimeOffset.Now.ToUnixTimeSeconds(),
  model = "model",
  system_fingerprint = "N/A",
  @object = "N/A",
  usage = (object)null
}));

To improve the experience around mocking, we plan to fully implement and expose the OpenAIModelFactory static class