Add new a2a call task to the specification and schema

[emmanuelallen]

Please specify parts of this PR update:

• Specification
• Schema
• Examples
• Extensions
• Use Cases
• Community
• CTK
• Other

Discussion or Issue link:

#1102

What this PR does:

Adds A2A call task to the specification and schema

Additional information:

Push Notifications
A2A push notification currently only support webhooks as a way to send task update notifications. That said external services can convert those webhook invocations into cloud events that the workflow can listen for. Hopefully at some point A2A can support natively or through an extension sending out push notifications as cloud events directly.

Streaming
I am suggesting streaming be handled by collecting up all the objects until the stream is complete and returning them as an ordered array. In the future we can add a foreach argument similar to AsyncAPI that when set will run a set of tasks for each object received.

[yzhao244]

@ricardozanini @cdavernas Our first PR which covers some of our discussions about A2A lol , please review and if necessary, we can even discuss further on our Thursday meeting :)