Add support for GoogleGenAi service_tier in GoogleGenAiChatOptions

[symphony-enrico]

Expected Behavior

Users should be able to specify the service_tier parameter when making calls to Gemini through Spring AI's GoogleGenAi. This parameter should be configurable via GoogleGenAiChatOptions.

Example usage:

GoogleGenAiChatOptions.builder().model(modelName).serviceTier(GoogleGenAiServiceTier.FLEX) // or STANDARD or `priority`

Current Behavior
Currently, there's no way for users to set this parameter when making API calls to Gemini through Spring AI.

Context

• Impact: Without this parameter, we cannot explicitly control the service tier for Gemini requests, which affects:
  • Cost management (different tiers have different pricing)
  • Performance characteristics (priority vs default processing)
  • Dedicated capacity usage (for enterprise deployments)
• Use Case: We need to specify service tiers for:
  • Production workloads requiring guaranteed capacity ("priority")
  • Cost-sensitive applications ("standard")
  • Dynamic scaling scenarios ("flex")

[symphony-enrico]

I created this draft PR #5899
However, it seems that latest version of GoogleGenAi has a bug and it will not works using Vertex Ai, so it is better to wait the fix before proceeding: googleapis/java-genai#972

[sdeleuze]

Closing in favor of the related PR #5899.