Insufficient Generative UI Integration

**Is your feature request related to a problem? Please describe.**
I'm always frustrated when building generative UI applications with ADK because there's no native support for UI state synchronization, artifact management, or streaming UI updates. Unlike mature frameworks like LangGraph that provide built-in state persistence and streaming capabilities, ADK requires developers to implement these patterns from scratch. The current ADK implementation lacks the foundational components needed for modern generative UI workflows where AI agents need to manage UI artifacts, handle real-time streaming updates, and maintain consistent state across user interactions.

**Describe the solution you'd like**
I would like ADK to include native generative UI integration features:

- Built-in artifact management system for handling UI components, code snippets, and other generated content
- Native SSE streaming support with UI state synchronization capabilities
- Integrated state management that can persist and restore UI context across sessions
- Pre-built components for common generative UI patterns (streaming text, progressive disclosure, artifact previews)
- APIs that seamlessly integrate with popular frontend frameworks (React, Vue, etc.)

**Describe alternatives you've considered**
Currently, I have to:

- Use the LangGraphAgent wrapper to leverage LangGraph's superior state management
- Implement custom streaming solutions on top of ADK's basic SSE support
- Build artifact management systems from scratch
- Manually handle UI state synchronization between frontend and backend

**Additional context**
Add any other context or screenshots about the feature request here.