Add vertexAi session service

[baptmont]

python ref: https://github.com/google/adk-python/blob/0ccc43cf49dc0882dc896455d6603a602d8a28e7/src/google/adk/sessions/vertex_ai_session_service.py

Fixes https://github.com/google/adk-go/issues/347

[gemini-code-assist[bot]]

Summary of Changes

Hello @baptmont, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the application's ability to manage sessions within Google Cloud's Vertex AI platform. It introduces a specialized Vertex AI session service, moving from a more generalized approach to a dedicated, API-driven implementation. This change provides a more structured and robust way to handle session lifecycle and event logging, ensuring better integration and scalability with Vertex AI reasoning engines.

Highlights

• New Vertex AI Session Service: Introduced a dedicated session/vertexai package to provide a robust session service implementation for Vertex AI, aligning with the Python ADK reference.
• Refactored Session Logic: The previous generic session.VertexAIService function has been removed and its functionality, along with detailed session management, is now encapsulated within the new session/vertexai package.
• Vertex AI API Integration: A new vertexAiClient has been added to handle interactions with the Google Cloud AI Platform API (specifically apiv1beta1), enabling creation, retrieval, listing, deletion of sessions, and event management for Vertex AI Reasoning Engines.
• Updated Example Usage: The examples/vertexai/agent.go file has been updated to utilize the new vertexai.NewSessionService and now requires environment variables (CLOUD_PROJECT_ID, CLOUD_LOCATION, VERTEX_ENGINE_ID) for configuration. A new example examples/vertexai/vertexengine/create_engine.go demonstrates how to create a Vertex AI Reasoning Engine.
• Dependency Updates: The go.mod and go.sum files have been updated to include cloud.google.com/go/aiplatform and google.golang.org/protobuf dependencies, necessary for interacting with the Vertex AI API.

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[baptmont]

missing state scopes currently only text events are stored

[baptmont]

/gemini review