Add OpenAI Go API example

[donvito]

Summary

• Add a standalone ai/go-openai module demonstrating OpenAI Chat Completions from Go
• Include a small reusable client and runnable main using OPENAI_API_KEY
• Add unit tests covering request construction, default model behavior, validation, API errors, and empty choices
• Link the new example from the root README

Testing

• go test ./... from ai/go-openai