[Feature Request] Attaching a file to a workitem

[AlexKeySmith]

The official documentation does not indicate how to upload attachments other than using the REST API directly.

Assuming az cli does not support it, it would be great to add support.

I can see create_attachment being referenced in the source code, but I gather it's in the SDK, but not exposed in the cli? I'm not a python guy, but I couldn't spot it.