Spring-ai feat(gemini) Support Safety Settings for VertexAiGeminiChatModel

Adds support for SafetySettings configuration to VertexAiGeminiChatModel and VertexAiGeminiChatOptions

See https://cloud.google.com/vertex-ai/generative-ai/docs/multimodal/configure-safety-filters for reference.

Should fix https://github.com/spring-projects/spring-ai/issues/942

Comment From: terussell85

@tzolov I updated the code with updates from the spring-javaformat plugin. Apologies that I didn't catch that the first time.

Comment From: terussell85

Great feedback. I'll take a look and get back to you.

Comment From: terussell85

@tzolov I think we're good to go now.

Changes since last review: 1. Introduce new VertexAiGeminiSafetySetting class to allow for declarative configuration. This is converted to the Vertex AI setting at request-time. 2. Not null assertions for safetySettings on both the Builder and the class property. 3. Verified that declarative configuration works 4. Updated docs

I made some changes to the originally suggested SafetySetting class, including renaming it and adding a Builder. I also kept the newly defined Spring enums both value- and name-consistent with the Gemini enums so that there won't be accidental confusion of users.