Add Text Embeddings Models in Gemini Plugin

[Xeftax]

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

Gemini offers a text embeddings API, but currently, Dify Gemini Plugin only supports the Gemini LLM.
Gemini API - Embeddings.

2. Additional context or comments

related issue: #15529

3. Can you help us with this feature?

• I am interested in contributing to this feature.