[Feat]: add support for the vertex managed task store service #751

Is your feature request related to a problem? Please describe.

No response

Describe the solution you'd like

It will be similar to src/a2a/server/tasks/database_task_store.py but use the vertex backend instead.

Describe alternatives you've considered

No response

Additional context

No response

Code of Conduct

• I agree to follow this project's Code of Conduct