AgentTool should support streaming back events from the child Agent to the parent

[ashubham]

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

Currently when using a sub agent as an AgentTool, only the final result of the agent is made available to the parent agent. Long running sub Agents, now have no way to stream intermediate results, or thoughts.

Describe the solution you'd like

AgentTool should support streaming back events from the child agent. Something like:

AgentTool(agent=sub_agent_1, stream_results=True)

This, should already be consumable by the Parent agent, as the ADK supports Streaming Tools.

Describe alternatives you've considered
Using sub_agents instead of AgentTool. But there we see the limitation that one sub_agent can only be a child of one parent. See Issue #4093

[sarojrout]

@ashubham , this #3991 should address this