feat(generative-ai): Add all modalities sample for Gemini 1.5

[gericdong]

Description

Fixes b/330500166

Add all modalities code sample for Gemini 1.5 Pro.

[snippet-bot]

Here is the summary of changes.

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• generative-ai/snippets/gemini-all-modalities.js:15, tag generativeaionvertexai_gemini_all_modalities

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