Stop Agent Execution button in ADK web

[zeroasterisk]

jules.google has implemented the request cancellation logic in agent.service.ts using an AbortController and added a stopSse method.

I have added the stop() method to chat.component.ts and connected it to the stop button.

The new stop functionality is implemented as a 1 shot by jules, and jules was not able to run the test suite.

This might be a concept more than it is a real PR, but capturing it as an artifact. Feel free to reject.

Related to https://github.com/google/adk-web/issues/9