Add VertexAI Session Service implementation

[kaugesaar]

Currently, adk-go lacks the VertexAI Session Service that exists in adk-python.

The Python implementation includes VertexAiSessionService which provides:

• Session creation, retrieval, listing, and deletion via Vertex AI APIs
• Event management with proper state handling
• Support for both standard and Express Mode authentication

I'm currently using the VertexAI Session Service in production with Python and need this functionality to migrate to Go. I'm willing to contribute the implementation. Would this be a welcome addition?

I can submit a PR implementing this service based on the Python implemenation.

[dpasiukevich]

Thanks for volunteering!

Vertex AI session service implementiation is WIP #235

[dpasiukevich]

Let me actually keep the issue open for visibility until #235 is merged.