Support for embedded UI components

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

The A2A protocol specification mentions potential support for "embedded UI components (e.g., iframes referenced in parts)" as a modality. Also, the [A2A demo video](https://storage.googleapis.com/gweb-developer-goog-blog-assets/original_videos/A2A_demo_v4.mp4) starting at 0:31 shows the agent protocol being used to negotiate the use of iframes, to display UI elements representing people.


After lots of searching, I couldn't find any implementation of this functionality in this repository's demo, or any other using A2A.

### Describe the solution you'd like

Adding support for displaying iframes/embedded UI components in the demo. Being able to display charts/tables/graphs, or any custom UI element like in the demo video, would be very useful for many applications using this protocol.

### Describe alternatives you've considered

_No response_

### Additional context

I'm not fully sure this issue should be in this repo, as opposed to in the A2A Protocol repo. If any maintainers think this issue would be more relevant there, or elsewhere, feel free to move it.

### Code of Conduct

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