[Feat]: Add source field in Message for multiple Agent discussion situation.

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

In the current A2A (Agent-to-Agent) protocol, messages are only defined with a kind of either User or Assistant. This design leads to the loss of critical information in multi-agent scenarios, such as collaborative or discussion-based workflows: specifically, which agent originated an Assistant message. This ambiguity can cause tasks executed in multi-agent environments to produce unexpected or incorrect results.

For example, in a multi-agent Werewolf game, if the messages lack a source identifier, the agents would be unable to perform analysis or make decisions based on who said what.

### Describe the solution you'd like

My current workaround is to add a `source` field within the Message's metadata to specify the origin. However, this approach is contingent upon the A2A server-side implementation. If a given A2A server does not support parsing this field, or uses a different field name for the same purpose, this capability is lost.

Therefore, I propose that the A2A protocol should natively support the attribution of message origins. This could be achieved by introducing a dedicated source field directly into the message structure. Adopting this change would provide all A2A servers with a unified standard for identifying a message's source, thereby ensuring compatibility and reliability in multi-agent collaboration and discussion scenarios.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

The issue is about extending the A2A protocol's Message structure. First, locate the protocol specification, likely in a schema or documentation directory. Understand the current message format and where new fields are defined. Review existing server implementations to see how metadata is handled. The change involves protocol design and ensuring backward compatibility, so 'done' means the spec is updated and example implementations reflect the new field.

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