[FEATURE] Add support of CancellationToken

[osdemah]

Problem Statement

As we're refactoring and adding asyncio support, it'd be great to add support of a "CancellationToken" throughout the SDK. This allows for convenient support of features like adding a stop/cancel button to the AI app. Sample implementation in autogen can be found here.

Proposed Solution

• requirement for this feature is to have asyncio support.
• Add CancellationToken to the agent entrypoint and pass it throughout the sdk (sample reference).
• Link the futures to the token.

Use Case

1. stop/cancel API for the AI app
2. complex multi-agent systems where the orchestrator agent can decide to stop the child agents

Alternatives Solutions

No response

Additional Context

No response

[zastrowm]

We will be exploring this as part of the async iterator support.

At first glance I think a lot of cancellation use cases can be solved by async iterators - e.g. just stop yielding - but where async iterators are not used (tool calls for instance) I think a cancellation-token would come into play.

Including this in #129 for design/exploration

[jer96]

Adding a comment in relation to #218 as the A2A protocol supports the notion of cancelling requests which may be relevant in the development of this feature. https://google-a2a.github.io/A2A/latest/specification/#74-taskscancel