[Feat]: Push Notification Payload can also be message, statusUpdate or artifactUpdate #490

Is your feature request related to a problem? Please describe.

According to the spec §4.3.3:

When a task update occurs, the agent sends an HTTP POST request to the configured webhook URL. The payload uses the same StreamResponse format as streaming operations, allowing push notifications to deliver the same event types as real-time streams.
[...]
The webhook payload is a StreamResponse object containing exactly one of the following:

• task: A Task object with the current task state
• message: A Message object containing a message response
• statusUpdate: A TaskStatusUpdateEvent indicating a status change
• artifactUpdate: A TaskArtifactUpdateEvent indicating artifact updates

At the moment only Task is supported by https://github.com/a2aproject/a2a-java/blob/main/server-common/src/main/java/io/a2a/server/tasks/PushNotificationSender.java.
The Java SDK must be updated to be able to push messages, status updates and artifacts updates in addition to the current task state.

Describe the solution you'd like

I want to be able to push messages, status updates and artifacts updates in addition to the current task state from the PushNotificationSender.

Describe alternatives you've considered

No response

Additional context

Afaic, the Python SDK does not allow to push messages, status & artifact updates (according to https://github.com/a2aproject/a2a-python/blob/main/src/a2a/server/tasks/push_notification_sender.py) and this part of the spec is not covered by the related tests at https://github.com/a2aproject/a2a-tck/blob/main/tests/optional/capabilities/test_push_notification_config_methods.py

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