Add Google A2UI

[muscariello]

We can leverage Google A2UI to visualize a number of things.

1. The current UI
2. A2A cards being used from agent directory especially when we enable tourist and guide discovery
3. Guide UI status: An human guide will use the UI to check on their phone the current scheduler which could be updated anytime. For instance last minute updates.
4. Tourist UI: same the UI will be updated live to bring the tourist in the right place at the right time.

W/ Google A2UI we can have several custom UIs but we can also have several custom backends of the UI acting as ADK agents. Even running locally.