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

I was looking for how to send the streaming "Message", and no solution than loading on Task or Artifact, and I feel both have problems in current state:

• Task: task update event will each chunk stored in the history as a single message, and single message is treated as final event, causing event queue closed
• Artifact: not plausible for someone's "Message"

Describe the solution you'd like

I tried to add 'isDelta' property in Message, so that event queue is only closed when isDelta is false or none, and merge each chunk in the last message's part of the history in the task when it has same message id.

Describe alternatives you've considered

No response

Additional context

I've tested this solution with task update event including new Message object with isDelta=True
I didn't concern grpc type, and tried not to affect other part, but tell me if it brings some problem

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