[Feat]: Built-in Pub/Sub support in the A2A protocol

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

_No response_

### Describe the solution you'd like

I've realized that asynchronous communication is highly helpful for agent communication. I also noticed that the Google ADK already supports agents communicating via Cloud Pub/Sub, as shown in this article: https://medium.com/@kamal.aboulhosn/vertex-ai-agents-cloud-pub-sub-8a11dc949246.

I would like to see native support for the Pub/Sub pattern in the A2A protocol. Are there any plans or roadmaps for implementing this?

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

### Code of Conduct

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