Support grounding for Vertex Gemini

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1. Is this request related to a challenge you're experiencing? Tell me about your story.

Currently Dify does not support grounding with Vertex Gemini. Grounding can improve the results massively so this would be a great feature.

Vertex:

2. Additional context or comments

No response

3. Can you help us with this feature?

• [ ] I am interested in contributing to this feature.