Add Vertex AI Support

[holtskinner]

Gemini, PaLM, Embeddings, Vector Search

• https://cloud.google.com/vertex-ai/docs/generative-ai/model-reference/gemini
• https://cloud.google.com/vertex-ai/docs/generative-ai/model-reference/text
• https://cloud.google.com/vertex-ai/docs/generative-ai/model-reference/text-embeddings
• https://cloud.google.com/vertex-ai/docs/vector-search/overview

[emrgnt-cmplxty]

This would be amazing, is this a PR that you could hep author @holtskinner? I am not on Vertex AI so testing would be a bit challenging.

[emrgnt-cmplxty]

Solved with LiteLLM integration.