feat: Support multi-modal moderations in openai_dart

[davidmigloz]

The moderations endpoint is a tool you can use to check whether an input is potentially harmful. Now it also supports image inputs:

final res = await client.createModeration(
  request: CreateModerationRequest(
    model: ModerationModel.model(ModerationModels.omniModerationLatest),
    input: ModerationInput.listModerationInputObject(
      [
        ModerationInputObject.imageUrl(
          imageUrl: ModerationInputObjectImageUrlImageUrl(
            url: 'https://upload.wikimedia.org/wikipedia/commons/9/92/95apple.jpeg',
          ),
        ),
        ModerationInputObject.text(
          text: 'I want to kill the apple.',
        ),
      ],
    ),
  ),
);
print(res.results.first.categories.violence);
// true