[Feature]: Support Custom Base URL Configuration for GeminiChatModel

[caichicong]

Feature Request: Support Custom Base URL Configuration for GeminiChatModel

Issue Description

Currently, the GeminiChatModel class does not support configuring a custom base URL, which limits flexibility for developers who need to:

1. Use proxy servers / API gateways to forward requests to Google Gemini/Vertex AI services
2. Connect to private, on-premises, or regional API endpoints
3. Test with mock services or local emulators

GeminiChatModel.builder()
                    .apiKey(apiKey)
                    .modelName(modelName)
                    .baseUrl(baseUrl)
                    .build();