Add support for Google Gemini

[markpollack]

Now that it is out ;) we can add support.

https://glaforge.medium.com/get-started-with-gemini-in-java-923f2069ea4d is a nice starter

[King-Chau]

Shall we add Gemini to spring-ai-vertex-ai module or a new module?

[King-Chau]

@markpollack #189 please review, thx

[knightcube]

I was just about to raise this issue. Good to see that it is already being worked on🙂

[tzolov]

Resolved with 65d42c9