OpenAI Go Plugin

[kekoawong]

Describe the solution you'd like
Create a go openai plugin per conversation wit@apascal07.

Additional context
Genkit value: getting structured data in and out. Core features of this plugin should be that we convert to/from genkit's structured format to OpenAI's format. Ensure that we are mapping this structure to/from the genkit and openai.

Requirements:
[ ] Genkit sample with the OpenAI plugin
[ ] OpenAI client (either custom or the official)
[ ] Support streaming for the plugin
[ ] Chat completions endpoint (we can start with text, move to other if necessary)