Agent Visualization

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

While developing and debugging multi-agent systems using the Agent Development Kit (ADK), it can be challenging to visualize the relationships and interactions between agents, tools, and handoffs. This lack of visualization makes it harder to understand complex agent architectures and to debug issues effectively.  

**Describe the solution you’d like**

I propose adding a built-in agent visualization feature to ADK, similar to the draw_graph functionality in the OpenAI Agents SDK. This feature would generate a structured graphical representation of agents and their relationships using Graphviz. Agents could be represented as yellow boxes, tools as green ellipses, and handoffs as directed edges between agents. Such a visualization would aid in understanding and debugging agent workflows. 

**Describe alternatives you’ve considered**

Currently, developers might manually create diagrams to represent agent relationships, but this is time-consuming and prone to errors. Alternatively, one could attempt to integrate external visualization tools, but a native solution within ADK would provide a more seamless and efficient experience.

**Additional context**

The OpenAI Agents SDK provides an example of such a visualization feature: https://openai.github.io/openai-agents-python/visualization/  

Implementing a similar feature in ADK would enhance its usability and align it with other agent development frameworks.