add litellm example (OpenAI-Compatible Endpoint)

[krrishdholakia]

Hi @random-forests @markmcd,

Adding a tutorial to show how to call Gemini through an OpenAI-Compatible Endpoint (via LiteLLM)

This supports Google AI Studio + Vertex AI as well.

Let me know if there's anything further required here!

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[MarkDaoust]

Hi, we're planning to put integration examples like this in an examples/integrations/ directory. Can you create that directory and move this file into it?

[MarkDaoust]

Also, se can't merge anything until you sign the CLA.

[krrishdholakia]

Hey @MarkDaoust thanks for the bump - just signed the CLA