feat: support pass through fetch options

[bytemain]

close: #8

Now you can use fetchOptions to diable credentials:

    const openai = new OpenAIClient({
      apiKey: bot.env.OPENAI_API_KEY,
      fetchOptions: {
        credentials: undefined,
      },
    });

now it works on CloudFlare Workers(have tested) , it actually overrided the ky request params: