[Feat]: Add token-by-token message streaming #1386

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

Right now it's not possible to stream messages token-by-token with A2A, resulting in a bad user experience.

I have built typical, conversational agents with A2A. They accept text, have a number of tools they can call, return the results of the tool calls as Artifacts, and send intermediate messages the agent generates as TaskStatusUpdateEvents.

However, because TaskStatusUpdateEvent doesn't have an append option, it's not possible to stream messages to the user token-by-token.

Because MessageUpdateEvent doesn't exist (and doesn't have an append option), it's not possible to stream standalone Messages to the user token-by-token.

Describe the solution you'd like

1. Add an append option to TaskStatusUpdateEvent to add token-by-token message streaming to Task Messages.
2. Create a MessageUpdateEvent with an append option to add token-by-token message streaming to standalone Messages.

Describe alternatives you've considered

It doesn't make sense to send intermediate messages the agent generates as Artifacts because they're not the output of the Task, they're commentary. It's important to keep Artifacts as they are because they effectively filter noise.

Additional context

No response

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