[Feat] Create a sample that demonstrates in-task authentication (secondary credentials)

We recently added a security sample that shows how an A2A client can send a message to a secured A2A server agent (see https://github.com/a2aproject/a2a-samples/tree/main/samples/java/agents/magic_8_ball_security).

It would be good to have a sample that demonstrates the case where an A2A server agent requires credentials for a different system or resource (e.g., to access a specific tool on behalf of the user that requires its own auth). See the "In-Task Authentication (Secondary Credentials) section [here](https://a2a-protocol.org/latest/specification/#45-in-task-authentication-secondary-credentials) for more details.

The new sample should be added to the a2a-samples repo.