Add Gemini support

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[Frankystew1]

Did Gemini get added?

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@Frankystew1 not yet!

[Thanzex]

The Claude 3 model family is very capable too, I would suggest changing this issue to make codel model-agnostic

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This is currently blocked by tmc/langchaingo#748.

While we can simply inject available tools in the prompt (as we're doing for Ollama, for example), it's not ideal since Vertex already supports function calling, and it would be better to just use it directly.