Document A2A super-task extension

We need to start some type of documentation on the various extensions we have made to A2A, so it is readable by other project partners.

For example, we could maintain an A2A-EXTENSIONS.md document in this repo, and write down any extensions as we develop them. I suggest we document at least their identifying URL, the extra metadata that they can attach to the various A2A messages, and the intended use / semantics of that extension.