Let pkg also be the home for common knative API and binaries

[yolocs]

Both serving and eventing start to have some functionalities that could benefit more than just the belonging repo. E.g. knative/eventing#2333 ConfigMap propagation API could be beneficial to both repos but the fact it only comes with eventing seems wrong. @grantr also mentioned that serving Certificate API could be another example.

pkg currently only have common libs. I wonder if could also be the home for common knative utility APIs and binaries?

[evankanderson]

We have a few common APIs in the "duck typing" package in apis today, so I think this has been done before.

In general, I think we'll end up needing to use duck typing to share these components.

[yolocs]

What about the controllers for such APIs? E.g. ConfigMap propagation. I think a single controller is good enough.

[github-actions]

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