Add Go OpenAI API example

[donvito]

Summary

• Add a standalone ai/go-openai example module using the OpenAI Responses API with Go's standard HTTP client
• Include a small main.go demonstrating OPENAI_API_KEY usage
• Add unit tests with an httptest server for request construction, response parsing, and error paths
• Link the new example from the root README

Testing

• go test ./... from ai/go-openai