feat(vertexai): token count

[Deleplace]

Prompt: "How many tokens are there in this prompt?"

Output:

There are 9 tokens in the prompt.

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Edit prompt "why is sky blue?"

Output:

There are 5 tokens in the prompt.

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Here is the summary of changes.

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• vertexai/token-count/tokencount.go:19, tag aiplatform_gemini_token_count

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[Deleplace]

@muncus Hi Marc, this PR is approved by Eli, could you merge it?