Spring-ai Add support for the Google VertexAI Embedding API

The new VertexAI Embedding API provides Text and Multilingual and Multimodal support. Implement a corresponding Text EmbeddingModel and explore the possibilities to leverage the multimodal API.

Comment From: dev-jonghoonpark

After realising that embedding is not available in gemini, I've been trying to make one since yesterday and you've already started working on it.

Thanks for the awesome work. I hope it gets merged soon.