Figure out the appropriate fields in v1beta1 SubscribableStatus.

[vaikas]

Describe the bug
As part of the work in:
#2373
and in particular: Subscribable duck type in the Subscribable.Status.Subscribers[] array, there was a question about if there's enough fields there to convey enough information and if the fields are aptly named. In order to make progress on the major parts, we're going to restrict that struct conversation to this issue.

@n3wscott

[vaikas]

This also applies in Sequence / Parallel, since they are exposed there in a similar fashion.

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