Spring AI [EPIC] - Support latest Google Unifed SDK to access Google AI Embedding Models

This is a placeholder EPIC issue to support https://ai.google.dev/gemini-api/docs/embeddings.

New embedding model release announcement https://developers.googleblog.com/en/gemini-embedding-available-gemini-api/

All previous threads/issues of discussion should be consolidated into this issue.

Comment From: misselvexu

/attempt ?

Comment From: ddobrin

I will provide a PR by end of week